Pipe table and table caption support (witiko/markdown backport)

[Omikhleia]

This PR is the backport to lunamark of Vít Novotný's pipe table support from the witiko/markdown project.
(https://github.com/Witiko/markdown)

Note: witiko/markdown is now LPPL-licensed, but these improvements were added when the project was still MIT-licensed.

See #31 for context.

I also added support for these to the HTML writer, so that to provide a non-regression test case. The HTML output mimics that from Pandoc in terms of output, class and styles.

[Witiko]

Nice work. I glanced over the changes and here is a minor nitpick: the extension is called pipe_table and claims to be named the same as Pandoc's extension for pipe tables. However, Pandoc's extension for pipe tables is called pipe_tables (plural).

[Omikhleia]

Nice work.

Thanks, although all credits are yours, it's quite a straightforward back-port ;)

However, Pandoc's extension for pipe tables is called pipe_tables (plural).

Oops, that's right. Shouldn't we follow the same name as in Pandoc, just for homogeneity? I.e. change it to pipe_tables?

(N.B. I'm leaving for vacations in the countryside tomorrow, not sure how network connectivity will be there... There's no urge, though!)

[Witiko]

Shouldn't we follow the same name as in Pandoc, just for homogeneity? I.e. change it to pipe_tables?

That's what I would do, but it's a minor complaint. Let me know when the PR is ready, so that I can do a proper review. Enjoy your vacation!

[Omikhleia]

Changed pipe_table option to pipe_tables

[Witiko]

Looks good, thanks again!