All commands now accept a version specification

Before this release dub could only get an exact version for some commands
(`describe`, `generate`, `fetch`, etc...). All commands now accept a version specification,
such as can be found in `dub.json` / `dub.sdl`:

    dub fetch 'foo@>0.2.0'
    dub describe foo@'>=0.3.0 <1.0.0'

Note that commands such as `describe` will still not fetch from the network.