Allow specifying specific projects.

    The code still defaults to looking at your watched projects configured
    in gerrit.  This allows you to specify a different list of projects
    to consider, instead.
    
    I am set up to watch most projects in gerrit, but when looking for a
    review, I usually only want to consider the projects for which I'm a
    -core team member.  So, now if I only want to see nova changes, I can do
    something like:
    
      $ next-review -u russellb openstack/nova openstack/python-novaclient