Fix/add compatibility with attr_list extension ids, classes

[hendrikp]

This fixes/adds the compatibility to supply custom anchor id, and class through the markdown text (through attr_list extension).

• I also fixed the readme in regard how generic caption (Listings) are html rendered.
• generic captions had a html-syntax errors previously in closing tag (contained class=listing statically)
  • this is fixed now

In regard to usage please see updated readme + unit tests.

Goal is to be able to be able to specify permalink / anchorid's to the resulting captions/elements in general through the attr_list extension. If no markdown attr_list specified the caption extension will still fallback to the previous id generation method.

[kyzyl]

Hi! Is this PR waiting on something? I was just considering adding something like this myself.

This is a great plugin, and this functionality would be very helpful to allow for automatic in-text cross referencing. I'm sure many people would appreciate it.

[hendrikp]

hm idk either, im using it from my branch for now - feel free todo the same. (install in pip through git source)

https://github.com/hendrikp/caption/tree/new-config-options-match-skip

[kyzyl]

Yep, me too. Thanks :) I hacked in some javascript to do automatic cross-referencing as well. It just replaces the <ref>{#fig-blah}</ref> tokens in text with the appropriate auto-numbered "Figure 1" etc. that is associated with the referenced ID. Can also add links to the IDs as well so readers can just jump straight to it. This way it works essentially the same as in LaTeX.

Of course this would probably be better done in the actual plugin source rather than hacking the DOM after the fact, but I didn't want to figure out how it works for now.