ttf-google-fonts-git PKGBUILD font selection option.

"provide=" and "install=" are touched up; pkgdir defined; added selection section.

I was hoping to spread this out into a couple edits, but I got a little lazy.  Sorry about
that.  Basically it's just cleanup with a font selection option available:

By default all fonts are installed, but if the user enters name in the
selection section then only those fonts will be installed.

Tested and it does work.