Multiple category support for components

This pull request adds the ability to sort a component into two top level categories, for instance having an auto complete input show up under both the category "form" and "javascript". We specifically wanted this so we could have an "all" category to preserve on-page browser text search while at the same time being able to break things into smaller categories for easier browsing.

- "category" accepts a comma separated list of categories or a single
  category
- preserves the name category in the doc blocks yml for backwards compatibility. That may end
  up seeming awkward, in which case we can change it, but it seemed fine
  for now.

Signed-off-by: Colin OByrne <cobyrne@pivotallabs.com>