Produce CSS in which local fonts are not used

[RLesur]

Hi @pvictor, @mfanny,

Thanks a lot for this beautiful gem (as usual :wink:)!

This PR is a proposal to allow the user to remove the local source which is here: https://github.com/dreamRs/gfonts/blob/7e921dfd587bdd3dbfe9bba9d24caf130cc3274a/inst/assets/templates/css_best.txt#L7

In fact, I have some use cases in which I need to remove the precedence of user installed fonts over downloaded fonts:

• user installed font version is different than the downloaded font version. In this case, I want to force the use of the downloaded font and avoid the use of a local font. The goal is that the web page looks exactly the same for each user.
• self contained R Markdown documents: since the font is embedded in the document, the use of a local source is needless.

[pvictor]

Nice! Thanks for the explanation :)