Create hg repo with --hg flag on cargo new

Allows the client to initialize a mercurial repo when creating a new cargo project with `cargo new --hg`. This would close #510.

This is my first time writing code in rust outside of a toy program so please feel free to tear it apart or suggest an alternative solution.
Hm, I'm not sure that we want to necessarily pull in `hg` as a dev-dependency for cargo just yet. I think it's ok for this to land without a test (it's pretty trivial)