Add a flag to turn on header ID generation.

[halostatue]

• Fixes TOC header ids #51, Create implicit header ids when no explicit ones are provided #101, and Human-readable anchor values for headings #102.
• Uses the code mentioned by @shurcooL from his Github
  Flavored Markdown parser extension in a comment on #102.
  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.

[halostatue]

When combined with #124, the automatically generated header ID will be used in the TOC. A manually provided header # header {#someid} will be used in preference to the automatically generated header.

[dmitshur]

I assumed that @shurcooL would be OK with this being included under the licence provided by blackfriday

I'm absolutely fine with that if it's helpful, no worries.

I did recently move that functionality out into a separate smaller package, so it's possible to import it without bringing in too many unrelated Go dependencies. See sanitized_anchor_name and shurcooL/go@c801d13.

[dmitshur]

The changes look good to me. Nice job adding tests too. It should close all those 3 issues.

But I'm not the best person to review this PR because this optionally (behind a flag) affects the HTML renderer, and I personally only use the blackfriday Markdown parser with my GFM renderer, which already does this functionality by default, so I'll never get a chance to use this.

[rtfb]

Thanks, @halostatue!