added support for installing a specific version

you can now install specific version from Nuget.
usage

 - `scriptcs -install microsoft.aspnet.webapi` - installs latest, like before
 - `scriptcs -install microsoft.aspnet.webapi -pre` - installs latest, including prerelease, like before
 - `scriptcs -install microsoft.aspnet.webapi 4.0.30506` - specific version
 - `scriptcs -install microsoft.aspnet.webapi -packageversion 4.0.30506` (version is reserved for scriptcs version) - alternative syntax

special versions are supported too
 - `scriptcs -install microsoft.aspnet.webapi 5.0.0-beta2`

Fixes #546