Allows for special styles to call them out since they're different than
normal text and different than code. They can make use of styles they
inherit for font style and weight.
Notes on changes:
- Added new CSS variables for reused elements
- The font-* rules are separate for each aspect so that they can inherit
bold/italic/etc
Closes https://github.com/rust-lang/mdBook/issues/1813
wkevin
Brett Chalupa
Yashodhan Joshi