Adds mixin for typography and defines defaults for ui, markdown, and compact markdown

After working with the current markdown scale we defined within the design library, @pedroms and I determined that there was not enough definition between headers for normal markdown files. For this reason, we've decided to split typography into three scales:

• UI: UI Typography consists of any type that exists within the UI that is not using markdown. This scale is needed in order to remove margins from typography, and to align the line height with other atoms.
• Markdown: Markdown consists within readmes, wikis, and documentation. This scale includes larger typography for more hierarchy, as well as margin-bottoms to separate the type.
• Compact markdown: Compact markdown is used on issuable pages (descriptions and comments). This scale includes a decreased scale compared to normal markdown typography, which appears too large within a normal UI page.

This approach is similar to what Github does with their markdown as well.

I've created a mixin to help with these three different defaults.