Support searching across groups of crates

[jsgf]

Some projects release their code in a lot of related crates - Tokio/Futures being the motivating example. It would nice to be able to group them all together and search them as a single entity.

I'm not sure where the info to define a group of crates would come from. Manual setting it one option, or the result of a crate search. Ideally it would be in the crates.io/crate metadata.

[Jayshua]

I came here looking for a similar feature. http://devdocs.io hosts a large selection of web-development related documentation. They allow the user to select which technologies they are interested in, and searching occurs accross all selected documentation at once. It's very convienent not to have to switch back-and-forth between lots of different documentation sources.

Maybe docs.rs could do something similar?

[jyn514]

Related: #494