Implementing tab autocomplete

Created by: siranipour

So @Zaharid and I thought it'd be cool to have tab autocomplete such that you could do

validphys --help <Tab> <Tab>

and get autocomplete functionality for all the VP providers, hence this PR. Currently only works for bash but I'll look into the implementing it for fish as well since that's what Zahari and myself use.

To get this working, in a bash shell, enter

complete -C ./autocomplete.py validphys

I don't really know where to put the autocomplete.py file. And to avoid having to type the complete command, we should really put that line in a file called validphys inside /usr/share/bash-completion/completions/ but I don't know how to do that automagically on install.

Finally, it may be better to compile a list of all the providers on install to avoid having to import the providers every time you press <Tab>