ENH missing decimal parameter in .to_latex and .to_html

[nbonnotte]

.to_csv has a decimal parameter to specify the decimal mark for floats since version 0.16

It would be nice to have the same thing in .to_latex and .to_html.

I'll work on a PR.

This issue is a spin-off of #7061, in which @TomAugspurger said:

There’s a possibility that we’ll be able to replace some of the to_latex code with a Jinja template, similar to the Style stuff. So don’t spend too much time on it :)

[jreback]

think we had 2 issues (#12030)