CodeMirror support. Improved image support.

[nawroth]

• Adds code to load Javascript libs dynamically. And execute them in the correct order.
• Adds CodeMirror support.
• Makes highlight.js load dynamically.
• Removes additional '/' present in some image URLs.
• Removes document name from image path in some cases.
• Tries to handle root-relative image paths. Fixes #17.
• Sets docgist environment variables. Fixes #9.
• Activates the toc macro by default.

[mojavelinux]

Looks excellent!

Makes me wonder if we should have CodeMirror support in Asciidoctor as well. I'm losing my mind a bit with all the syntax highlighting options already, but JavaScript ones are easy to support so perhaps it isn't too much of a headache.

[mojavelinux]

Is CodeMirror much better than the alternatives?

[nawroth]

CodeMirror is different as you can use it for editors as well. That's what we use in the Neo4j web UI, and thus I wanted to use it for other syntax highlighting too. No idea about which one is better really.

[mojavelinux]

Aha, that makes sense. You use it for consistency across the interface. Now I understand.

I still wonder whether it makes sense to integrate into Asciidoctor core. If so, perhaps you can file an issue and we'll look into it.

There's also an outstanding request for Prism. I sort of wish the JavaScript-based highlighters would align a bit.

[nawroth]

See also #17

[nawroth]

@mojavelinux Sorry for this "all the things" commit/PR. If we want to revert part of it, I can do that by hand.

[mojavelinux]

Not a problem at all. While finer grain commits are ideal, sometimes getting it all in there takes precedence. Onwards!