Add optional arguments to config

I'd be interested in some feedback on the API here, but I've added the ability to specify optional arguments when launching a project. You add them as ERB blocks and reference @argv[i] in the template. Some example use cases:

You may have a lot of rails projects that vary in name but the basic project structure is the same. You may also want to start the servers on a different port.

Arguments are specified after the command like mux start myproject arg1 arg2 arg3. Does not affect the way non-parameterized projects are started.