Local vendor and gpm exec

I created a gpm plugin https://github.com/hybridgroup/gpm-exec for locally vendoring packages and executing Go commands in the context of the locally vendored packages, but I think this functionality would be incredibly useful in the core package.

This pull requests adds an optional `--path` parameter to the `gpm install` command which allows you to specify the location of the vendor directory. It also adds a `gpm exec` command which executes Go commands in the context of the vendor directory.