Add '.gitignore' to new projects

[fraya]

When a new library or application is created, create a '.gitignore' configured for the OpenDylan language.

[cgay]

In general I don't want deft to depend on a specific version control system; it should work equally well with Mercurial, Git, Perforce, etc. (The fact that it currently downloads packages via Git is an implementation detail, used for expediency, that I expect to remove at some point.)

That said, I think this is fine, and we can add a flag later to decide whether to generate .gitignore, .hgignore, etc.