Simplify API

Two breaking changes in interest of simplifying the UI.

• baseLineHeight takes the css unitless number now instead of a px value. This is what most people use for line-height so made sense to make the switch. PX values still work so this isn't technically a breaking change but you're encouraged to switch things over.
• modularScales is now just scale. I decided to consolidate various responsive typography concerns into a coming breakpoints option. Instead of passing an array of options to modularScales do something like:

{
  scale: 2
}