read RUST_DOC_PATH and add hyperlinks to the relevant parts

[infinity0]

It would be really nice if I could click on each auto-completion suggestion and read their documentation. (Or in text mode, perhaps instead hit some key and just read it from inside emacs.)

It should be possible for RUST_DOC_PATH to either be a URL (e.g. to the online rust docs) or a local path (e.g. for rust-doc).

[ane]

company has support for this, there's a possibility for adding documentation for each completion item, but racer itself doesn't read them (yet). So we need to implement that feature into Racer first, after that is done, implementing the feature is trivial.

[nickbroon]

I think https://github.com/phildawes/racer/issues/415 might be tracking adding the necessary support in racer.

[Wilfred]

Completed as part of #46.