Override the yard doc generate to fix hash links in the readme

The readme uses GitHub hash links which go something like
like #another-section and yard generates hash links that
go something like #Another_section.

This changes the yard generator to make the transformation to
capitalized underscored hash links so they work both on GitHub
and in the yard generated documentation.
Fix all the hash links instead of only the first match
Merge pull request #478 from octokit/yard-custom-readme

Override the yard doc generate to fix hash links in the readme