List revisions for given service

[navidshaikh]

Presently, kn revision get will return all available revisions. kn should be able to filter the revisions for a given service, for eg: kn revision get --service svc1.

[navidshaikh]

The fix should also update this section of docs as well.

[rhuss]

FYI, #75 introduced a client facade which has a GetRevisionsForService method.

[rhuss]

#75 also implements the listing of all revisions as part of kn service describe (see screenshots)

[navidshaikh]

/assign navidshaikh