Allow configuring the docker image of DocumenterLaTeX

[findmyway]

Currently the docker image and tag are hard-coded in the source code:

https://github.com/JuliaDocs/Documenter.jl/blob/28de68c0a10c7f7e8f7e6de9dcf266f9ab5947e1/src/Writers/LaTeXWriter.jl#L134

https://github.com/JuliaDocs/Documenter.jl/blob/28de68c0a10c7f7e8f7e6de9dcf266f9ab5947e1/src/Writers/LaTeXWriter.jl#L159

It would be very convient to allow configuring them.

[mortenpi]

As an argument to LaTeX() perhaps? @fredrikekre do you have any thoughts on this?

[fredrikekre]

As an argument to LaTeX() perhaps?

Sure, that makes sense.