Allow use of a function for fig, eq or tab labels

[cderv]

This will solves #1114

For now I only added support for fig, eq and tab. Support for theorem or proof env is a bit trickier because label_prefix is used for find the label to use for \newtheorem in preamble where number is not used

bookdown/R/latex.R

Line 239 in cec8efb

str_trim(vapply(theorem_abbr, label_prefix, character(1), USE.NAMES = FALSE)),

and for proof env too

bookdown/R/latex.R

Line 248 in cec8efb

gsub('^\\s+|[.]\\s*$', '', vapply(proof_envs, label_prefix, character(1), label_names_math2))

This means it can't be a function of reference number, and case is different between LaTeX and non LaTeX.
I wanted to support all because for non LaTeX output this is the same mechanism: label is prepended by default. However, I did not find an easy way to do it because of the constraint mention above. But also I am not sure that what I have done currently is a lot easier.

@yihui you may want to review that more closely in case you see a way to also allow the use of a function for all theorem_abbr and label_names_math2.

[cderv]

It is not part of the feature request but I think it was not working as expected before, and the span was never inserted. So I fixed it here.

[yihui]

Sounds good. Thanks for catching it!

[yihui]

I think it is much more common for users to customize figure and table names. We can consider theorems in the future if someone asks for the feature. Thanks!

[yihui]

Sounds good. Thanks for catching it!

[tamas-ferenci]

Thank you! Unfortunately theorem is the same story in Hungarian (Theorem 1 should be 1. tétel), but it is of course needed much-much less often than figure or table, and I am personally fine now. Thank you again!

[cderv]

Yes theorem was trickier to support.
Do you change it already for pdf using LaTeX in preamble ?