Add command to scaffold a new mono_repo

[sarahec]

dart-lang/stagehand#612 asks the musical question: can we make it easier for people to set up mono_repo.

For devs who aren't setting up Travis integration (using only mono_repo pub get and mono_repo pub upgrade), the setup involves creating a set of blank or single-comment text files. It's a minor annoyance when retrofitting a project and could be automated easily.

Proposal: Add a mono_repo init command that scans for the required files and generates them if needed (akin to git init).