Improve parsing of tex strings

[goerz]

Implements a proper (albeit limited) single-pass parser for the subset of tex syntax that occurs in BibTeX fields, converting to the equivalent Markdown. Most importantly, this handles accents like \"o, and groupings, i.e., it removes curly braces in the same way that LaTeX does.

Inline math is recognized and translated into Documenter's preferred "``" syntax, but otherwise is left alone.

A limited set of LaTeX commands is recognized, most notably \href and \url, allowing to put hyperlinks in the @note field of a reference. We're deliberately conservative here. Anyone who can demonstrate the need for additional commands should submit an issue with a real-life example. It should be quite trivial to add any specific additional command.

All of this is considered "very internal". We're not making any promises. Ideally, a lot of this code would probably be better off in Bibliography.jl, but at this point, we're treating Bibliography as just a basic parser to get the raw tex, and doing all the processing / conversion to Markdown ourselves.

Closes #15.

Should also address #44, except that the lack of a new BibInternal release still requires the workaround of using non-breaking spaces.

[goerz]

The \url/\href support probably shouldn't be overused, but the rendering of links as in GoerzPhd2015 can be pretty neat, from the following .bib entry:

DocumenterCitations.jl/docs/src/refs.bib

Lines 278 to 285 in 8f83b48

	@phdthesis{GoerzPhd2015,
	Author = {Goerz, Michael},
	Title = {Optimizing Robust Quantum Gates in Open Quantum Systems},
	School = {Universität Kassel},
	url = {https://kobra.uni-kassel.de/handle/123456789/2015052748381},
	Year = {2015},
	note = {See \url{https://michaelgoerz.net} for additional formats. Source available on \href{https://github.com/goerz/dissertation}{Github}},
	}

[goerz]

@kellertuer @Seelengrab @lgoettgens Can you have a look?

Are there any specific LaTeX commands that you would like me to add support for?

This should be the last PR that goes into the 1.3 release. It should also make DocumenterCitations pretty feature-complete, as far as I'm concerned. Thus, after the 1.3 release, I'd wait a while to see if anything comes up from the plugin being used in the wild, and then at some point release 1.4 that declares the documented Internals as "stable", as discussed in #43 (comment).

[kellertuer]

I am fine with the current state of rendering I think.

[Seelengrab]

Anyone who can demonstrate the need for additional commands should submit an issue with a real-life example. It should be quite trivial to add any specific additional command.

I was wondering whether supporting urldate would be ok? It's not technically bibtex if I'm not mistaken, but does give online ressources a better anchor.

Other than that, I'm very happy already with the current state, so nothing for me to add either :)

[goerz]

I was wondering whether supporting urldate would be ok?

Can you open a separate issue for that, preferably with an example BibTeX entry, and how it should render?

[goerz]

@Seelengrab Opened #53, we can discuss urldate there.