Add scss-lint configuration

This is a clone of thoughtbot's configuration from our guides
(https://github.com/thoughtbot/guides/), with the addition of
BEM-related linters enabled.

.hound.yml

@@ -2,3 +2,6 @@ fail_on_violations: true
 
 ruby:
   config_file: .rubocop.yml
+
+scss:
+  config_file: .scss-lint.yml

.scss-lint.yml

@@ -0,0 +1,251 @@
+scss_files: "app/assets/stylesheets/**/*.scss"
+
+severity: warning
+
+linters:
+  BangFormat:
+    enabled: true
+    space_before_bang: true
+    space_after_bang: false
+
+  BemDepth:
+    enabled: true
+    max_elements: 1
+
+  BorderZero:
+    enabled: true
+    convention: zero
+
+  ChainedClasses:
+    enabled: true
+
+  ColorKeyword:
+    enabled: true
+
+  ColorVariable:
+    enabled: true
+
+  Comment:
+    enabled: true
+    style: silent
+
+  DebugStatement:
+    enabled: true
+
+  DeclarationOrder:
+    enabled: true
+
+  DisableLinterReason:
+    enabled: false
+
+  DuplicateProperty:
+    enabled: true
+
+  ElsePlacement:
+    enabled: true
+    style: same_line
+
+  EmptyLineBetweenBlocks:
+    enabled: true
+    ignore_single_line_blocks: true
+
+  EmptyRule:
+    enabled: true
+
+  ExtendDirective:
+    enabled: true
+
+  FinalNewline:
+    enabled: true
+    present: true
+
+  HexLength:
+    enabled: true
+    style: short
+
+  HexNotation:
+    enabled: true
+    style: lowercase
+
+  HexValidation:
+    enabled: true
+
+  IdSelector:
+    enabled: true
+
+  ImportantRule:
+    enabled: true
+    exclude:
+      - "app/assets/stylesheets/utilities/**"
+
+  ImportPath:
+    enabled: true
+    leading_underscore: false
+    filename_extension: false
+
+  Indentation:
+    enabled: true
+    allow_non_nested_indentation: false
+    character: space
+    width: 2
+
+  LeadingZero:
+    enabled: true
+    style: include_zero
+
+  LengthVariable:
+    enabled: false
+
+  MergeableSelector:
+    enabled: true
+    force_nesting: true
+
+  NameFormat:
+    enabled: true
+    allow_leading_underscore: true
+    convention: hyphenated_lowercase
+
+  NestingDepth:
+    enabled: true
+    max_depth: 3
+    ignore_parent_selectors: false
+
+  PlaceholderInExtend:
+    enabled: true
+
+  PrivateNamingConvention:
+    enabled: true
+    prefix: _
+
+  PropertyCount:
+    enabled: false
+
+  PropertySortOrder:
+    enabled: true
+    ignore_unspecified: false
+    min_properties: 2
+    separate_groups: false
+
+  PropertySpelling:
+    enabled: true
+    extra_properties:
+      - font-variant-numeric
+      - text-decoration-skip
+
+  PropertyUnits:
+    enabled: true
+    global: [
+      'ch', 'em', 'ex', 'rem',
+      'cm', 'in', 'mm', 'pc', 'pt', 'px', 'q',
+      'vh', 'vw', 'vmin', 'vmax',
+      'deg', 'grad', 'rad', 'turn',
+      'ms', 's',
+      'Hz', 'kHz',
+      'dpi', 'dpcm', 'dppx',
+      '%']
+    properties:
+      line-height: []
+
+  PseudoElement:
+    enabled: true
+
+  QualifyingElement:
+    enabled: true
+    allow_element_with_attribute: false
+    allow_element_with_class: false
+    allow_element_with_id: false
+
+  SelectorDepth:
+    enabled: true
+    max_depth: 3
+
+  SelectorFormat:
+    enabled: true
+    convention: hyphenated_BEM
+
+  Shorthand:
+    enabled: true
+
+  SingleLinePerProperty:
+    enabled: true
+    allow_single_line_rule_sets: true
+
+  SingleLinePerSelector:
+    enabled: true
+
+  SpaceAfterComma:
+    enabled: true
+    style: at_least_one_space
+
+  SpaceAfterComment:
+    enabled: true
+    style: at_least_one_space
+    allow_empty_comments: true
+
+  SpaceAfterPropertyColon:
+    enabled: true
+    style: at_least_one_space
+
+  SpaceAfterPropertyName:
+    enabled: true
+
+  SpaceAfterVariableColon:
+    enabled: true
+    style: at_least_one_space
+
+  SpaceAfterVariableName:
+    enabled: true
+
+  SpaceAroundOperator:
+    enabled: true
+    style: at_least_one_space
+
+  SpaceBeforeBrace:
+    enabled: true
+    style: space
+    allow_single_line_padding: false
+
+  SpaceBetweenParens:
+    enabled: true
+    spaces: 0
+
+  StringQuotes:
+    enabled: true
+    style: double_quotes
+
+  TrailingSemicolon:
+    enabled: true
+
+  TrailingWhitespace:
+    enabled: true
+
+  TrailingZero:
+    enabled: true
+
+  TransitionAll:
+    enabled: true
+
+  UnnecessaryMantissa:
+    enabled: true
+
+  UnnecessaryParentReference:
+    enabled: true
+
+  UrlFormat:
+    enabled: false
+
+  UrlQuotes:
+    enabled: true
+
+  VariableForProperty:
+    enabled: false
+
+  VendorPrefix:
+    enabled: true
+    identifier_list: base
+
+  ZeroUnit:
+    enabled: true
+
+  Compass::*:
+    enabled: false