Flag: extend-css

This issue is part of https://github.com/steveklabnik/rustdoc/issues/125

-e/--extend-css: extend rustdoc's CSS

Using this flag looks like this:

$ rustdoc src/lib.rs -e extra.css
$ rustdoc src/lib.rs --extend-css extra.css

With this flag, the contents of the files you pass are included at the bottom of Rustdoc's theme.css file.

While this flag is stable, the contents of theme.css are not, so be careful! Updates may break your theme extensions.

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Should we keep this flag or not?