Add ability for rustdoc to use custom CSS.

[SiegeLord]

Proposed API:

--custom-css MAIN_CSS [AUX_CSS...]

MAIN_CSS would be copied as main.css into the output directory and linked to by all the HTML files. The set of other CSS files specified by AUX_CSS will be copied into the output directory, but not linked to. The intention is for the MAIN_CSS to @import them (like is currently done to normalize.css).

Alternatively, they could all be linked in the HTML files in some specified order (the internet suggests linking is better than @import).

[steveklabnik]

This is already possible with --markdown-css.