Different anchor tags for some headings.

[dmitshur]

The GitHub API and this Go package generate slightly different anchor tags for headings with periods, and some other sequences of special characters.

This is because of a difference in algorithm for escaping of heading anchor names in GFM when rendered by GitHub API, compared to the behavior of sanitized_anchor_name that this package relies on.

For example, GitHub turns "Date & Time" to "#date--time",
but this package escapes that as "#date-time".

It would be very helpful if there was a formal specification for the algorithm GitHub uses, rather than having to guess what it is. GitHub released a formal spec for GFM, but I couldn't find a section covering this there. I tried asking at https://twitter.com/shurcooL/status/847212011649937408, but no response there.

/cc @dominikh