Add support for Hungarian language @aref

[dmurdoch]

As mentioned in Issue #671, the babel Hungarian support in LaTeX adds the macro \aref to be like \ref but with support for some special Hungarian grammatical constraints.

This PR takes the changes from my magyarbookdown package and includes them in bookdown. I'm not sure this is an appropriate addition, but I'll let you decide.

[CLAassistant]

All committers have signed the CLA.

[yihui]

With the recent PR #1120 (aiming to fix #1114), it's possible to customize figure labels via a function provided by users. I don't know anything about Hungarian, but I guess at least it should be possible to generate a label like az 1. ábra now for non-LaTeX output. If that's the case, we don't need to embed the label-generating logic for a particular language inside this package.

For LaTeX output, perhaps we just need a config option for users to specify which LaTeX command they'd like to convert \@ref() to (the default would still be \ref{}). Again, I know nothing about \aref{}, so my naive question is, could redefining \ref to \aref solve the problem?

Thanks!

[tamas-ferenci]

Thank you! Unfortunately this redefinition wouldn't really solve the problem, as we might need \ref as well. (aref is only need when there is a definite article in the reference text.) If possible, the best would be to introduce \@aref identically to \@ref, with the former translating to aref in LaTeX, the latter to ref. Of course, HTML output also needs its own logic, similar to https://github.com/dmurdoch/magyarbookdown/blob/master/R/replacements.R .

[yihui]

@tamas-ferenci Thanks for the explanation! We'll think more about it.