Handle standalone images as fragmented content

This now works as expected:

[[./my-image.png]]
Merge pull request #18 from legoscia/fragment-image

Handle standalone images as fragmented content.
Thanks for Magnus's codes.
feat: add variables for custom title page

REVEAL_TITLE_IMG/REVEAL_TITLE_IMG_WIDTH/REVEAL_WEBPAGE/REVEAL_VENUE
feat: add support "BEGIN_CODE" and "END_CODE" to use highlight.js
feat(ox-reveal): add REVEAL_TITLE_TOP_MARGIN