Add animation and keyframes mixins
Add font-size mixin with arg for adjusting line-height (default) based on base line height and font size.
Add and import new headings and variables stylesheets.
Update site styles to start using consistent vertical rhythm.
Use h2 for about-me-short.
Change nav current class name to is-selected.
Add base spacing margin-bottom to ul/ol.
Flatten selectors so we don't have deep nesting. Keep it simple with clear class names.

I find this approach to be a lot better, more flexible and easier to
debug. Finally had the time to clean it up.
Add variables for base-text-color, base-font-family and heading-font-family.

Further remove custom colors on code blocks as we want to use the same
body text color.
Reuse post-header class so headers appear as they do elsewhere.