First pass at contextual display for `<code>css-property</code>`s
Improvements to contextual display:

1) Mobile support
2) Fix horizontal scroll issues
3) Better highlight of `.contextual-open-tree`
Make necessary DOM adjustments to other chapters to support contextual display
Better highlighting of context
Better contextual styling
More term definitions
Merge pull request #2 from adamschwartz/contextual-display

Contextual display