Add options to customize download

[vermiculus]

Personally, I'm only interested in a --source-only, but I imagine others might have use for all the fonts, etc. in their testing.

Fonts and documentation are roughly 1.5GB each. For some purposes (Travis, in my case), they aren't worth downloading.

See http://tex.stackexchange.com/q/119759/17423 for a breakdown of the distribution size.

[scottkosty]

@vermiculus I'm sorry for my embarrassingly late reply. I'm guessing you've moved on to greener pastures, but if not let me know. Is "--source-only" an argument for install-tl?