Fix output to be more similar to GitHub

• SHA references are now wrapped in inline code:

Before: [1f2a4fb](1f2a4fb) After: [`1f2a4fb`](1f2a4fb)

This also works for User@SHA and User/Project@SHA.

• Mentions are now wrapped in strong emphasis:

Before: [@wooorm](https://github.com/wooorm) After: [**@wooorm**](https://github.com/wooorm)

Note that this isn’t exactly like GitHub, as they add a custom
CSS class to remove the link-color from these mentions, which
is sanitised when trying to insert it from markdown.

• Remove distribution files from source (10d4614)
• Remove support for bower (bf06b8e)
• Refactor to replace mocha with tape (a6318c0)
• Replace internal dependency of mdast > remark (384629d)

• 7082f35 Rename mdast-github > remark-github
• bbcf85b Refactor for changes in mdast

🎉