Add a flag to turn on header ID generation.

- Fixes #51, #101, and #102.
- Uses the [code][gfm] mentioned by @shurcooL from his Github
  Flavored Markdown parser extension in a [comment on #102][comment].
  Since this was mentioned, I assumed that @shurcooL would be OK with
  this being included under the licence provided by blackfriday (there
  is no licence comment on his code).
- I’ve added it behind another flag, EXTENSION_AUTO_HEADER_IDS, that
  would need to be turned on for it to work. It works with both prefix
  and underline headers.

[gfm]: https://github.com/shurcooL/go/blob/3bec0366a85101d116bf88b477e92ea6cdd75e4e/github_flavored_markdown/main.go#L90-L102
[comment]: https://github.com/russross/blackfriday/issues/102#issuecomment-51272260