Add new --extend-css option to rustdoc

[GuillaumeGomez]

The purpose of this option is to provide a css-file to rustdoc while it's generating documentation to refine some css rule. For example, if you want to have a dark-theme for the generated doc, you'll provide a css file which replaces css rules defined in this file.

> rustdoc --extend-css dark.css your_project

cc @brson