Add --roots option to the "install" command

[thaljef]

Effectively, this is a shortcut for:

pinto roots | pinto install

Note: For remote repositories, this will require a custom Action class that will first fetch the roots, and then do the installation. This is because we need the list of roots on the client side so we can give them to cpanm.

[thaljef]

Per discussion in #158, the roots command is doomed. So this proposal is no longer feasible.

Instead, I'm going to focus on making the "main module" feature work better, which should produce a better result anyway.