Typography refactor

# Typography Refactor

## Key Points:

- Font settings to define key variables upfront;
- Non-hierarchical (temporarily named after Greek mythological characters) font size variable names;
- Simplified mixins for controlling font-size, font-weight and line-height;
- A base-heading-style to zero all top margins and set a uniform line-height of 1 to prevent awkward wrapping;
- A base-font-style that responds to set a desktop size of 18px;
- A base-button-style with font-smoothing;
- Simpler heading classes that can be applied to HTML or extended.
I think the phonetic alphabet is easier to follow: https://github.com/mish-mash-style/mishmash#instance-modifiers