Add an edit subcommand

[brianp]

Add a subcommand that will open the users default editor to make changes on the supplied project file.

Ex.

$ muxed edit my_project

Where vi would open to edit the file ~/.muxed/my_project.yml

The subcommand should also accept -p as an argument for path where to find the file.
Ex.

$ muxed edit my_project -p ~/.my_muxed_projects/

Where vi would open to edit the file ~/.my_muxed_projects/my_project.yml