Allow subdirectories within git repos in npm install 

I would like to be able to run `npm install` for a `package.json` file in a subdirectory of a git repo.

This would be useful for projects that use releases systems (such as [nzakas/parserlib](https://github.com/nzakas/parser-lib/blob/master/release/npm/package.json)), and for git repos that contain multiple npm packages .

Can this be added?
