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.

[ferenci-tamas]

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.