tools: add .editorconfig (SuperTux#424)

Allows consistent configuration of indentation and encoding rules across
different editors (http://editorconfig.org/).

[ci skip]

Author: maxteufel

.editorconfig

@@ -0,0 +1,13 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+max_line_length = 80
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false