Add the ability to define sh/exec commands

[svrana]

For example, it might be nice to have a 'dev python' command which runs 'dev sh python foo.py'. Adding code for each of these cases shouldn't be necessary. Add the ability to define these shortcuts globally or per-project in the configuration file.

I'm thinking that a command defined outside of the project would apply to all projects, i.e., in the case above, dev projectname python.

Possible configuration (needs work)

sh_shortcuts: {
   "python": "/home/app/.virtualenv/bin/python",
}

[svrana]

Punting on this. Probably a better fit for short scripts living outside the tool. We'll see.