Add header anchor

[zenorocha]

metaljs.com has a nice little feature of having anchors on the headers.

This was added manually on the markdown level:

## [Alias](#alias)

On GitHub's readme, they also offer that functionally, but their markdown renderer automatically adds an anchor with ID on the headers.

We could research if Remarkable (our Markdown renderer) has that feature and add it to our markdownOptions settings.

[justinsunho]

Here are two threads related to this issue. It looks like the feature isn't built in, and you'd have to create a separate plugin to achieve this.

jonschlinkert/remarkable#122

jonschlinkert/remarkable#284