Permit words for use as section keys.

Nothing in the codebase sorts by number as far as I can tell,
and this would allow for the flexibity should you want it instead
of numbers.  Also would keep you from having to keep renumbering
if you reorder top level elements.

Additional comments from squashed commits:

- Add parser tests to check keys like Buttons.Big
- Fix regex and test for word phrase as section name
- This permits a section name like 'Buttons - Crazy Rainbow'
- Add a section test for a word phrase as reference
- Update readme and spec regarding word section refs
Merge pull request #65 from bemurphy/word_hierarchy

Permit words for use as section keys.