Add command to vendor dependencies.

This PR adds `./gp vendor` to vendor the project dependencies under your favourite scm.

This is the internal process to do this:

1. Fetch missing dependencies, if any.
2. Remove all the `.git, .hg...` files from the dependency directories.
3. Create ignore rules for the project's scm to ignore `.gopack/vendor/bin` and `.gopack/vendor/pkg`.
4. Modify your `gopack.config` to tell gopack that the dependencies are vendored.

/cc @d2fn, @jingweno