Dependency update: markdown-it 13, mini->micromatch, etc.

[zackw]

This plugin hasn't had its dependencies updated in quite some time and npm audit is quite cranky about it. Here's a PR which updates everything. Note in particular the change from minimatch to micromatch, which was not strictly necessary but aligns behavior better with metalsmith core. All tests pass.

As you can probably tell from the other two commits in this PR I have some pretty strong opinions about how things should be done, let me know if any of that doesn't suit you.

[zackw]

Would close #19.