Support peerDependencies in the no-extraneous-dependencies rule

[benmosher]

Analogous to #266. Ideally should be implemented along the same rails.

[jfmengels]

I'm not too familiar with peerDependencies and their use. Is having a peerDependency a good excuse of not setting some package in your dependencies as you assume that it will be installed in the project using your package ?

I can add this. Should we have the option for this be true or false by default?

[benmosher]

false by default, let people enable. Might be good to explicitly state in the warning for false mode that the referenced package is in peerDependencies.

Good example is React/ component relationships. React component packages will need to import React from 'react' but need to get the project react module, not some local one. So they (should) declare the react dependency as a peer.