Merge pull request #1 from codepiano/ignore_empty_and_string

ignore empty object and plain string in function tok()
Add tests for footnotes
Add footnote parsing/rendering support

Fixes GitbookIO/gitbook#395, fixes GitbookIO/gitbook#134, fixes
GitbookIO/gitbook#237

Based off https://github.com/chjj/marked/pull/431
Merge pull request #2 from GitbookIO/feature-footnotes

Feature footnotes
Bump version to 0.4.1
Add negative inline math tests

Markup that looks like math (contains dollars) but isn’t really
Use new kramdown convention for inline math

$$ x $$
rather than
$ x $

This also helps avoid headaches for authors of financial documents/books
Update inline math tests for new convention
Bump version to 0.4.2