Support MongoDB URIs as mongo shell argument

It would be more consistent with the drivers if the shell supported URIs, in addition to --host replSetName/seed-list format it already supports.
E.g.

mongo mongodb://test:test@127.0.0.1:27017/test

Original description was:
The shell appears not to be able to take mongodb:// urls as elements of argv.
The shell appears not to be able to connect to a replica set specified as <rsname>/host1[:port1][,host2:[:port2]...] as elements of argv. Either of these would be nice, but if we do just one, the first is better than the second, as the double-slashes in the URL are already an error for the shell's idiosyncratic connection syntax.