add background support.
Add HTML block support.
- generated markup is now well formed XML

- XML doc types should be uppercase.

- added a few new lines so that all elements are on their own line
Merge pull request #17 from skybert/master

- generated markup is now well formed XML
feat: add variables for custom title page

REVEAL_TITLE_IMG/REVEAL_TITLE_IMG_WIDTH/REVEAL_WEBPAGE/REVEAL_VENUE