Enable --random flag by default

MultiMarkdown provides a --random argument that causes it to generate random footnote IDs instead of sequential ones, useful in cases where multiple snippets (that use footnotes) are rendered to a single page (such as a blog).

This pull request enables this behavior. Though the enums defined in extensions.hpp have changed slightly, they're straight from the same MultiMarkdown version (4.4.2) used by node-mmd (i.e. they were a bit out of date in extensions.hpp).