add '--server arg' to install command

I add a '--server' argument to make it possible for user to specify just ONE custom repository for module installation.
This is useful when one doesn't want to retrieve all official repositories or ones in the "repositories" file .This is also useful if  there's collision between modules in your repositories and the default ones.

You can specify the repository like this:
'play install my_module-version --server=http://myrepo'

BTW, I change the regexp for module name to allow '_' to be part of the module name.