Provide a way to externally provide overriding aliases #39

Imagine the case where you have a build environment disconnected from the Internet and using a mirror or mirror like server. Now you have a project with an existing glide yaml file. To point to the mirror should that be a fork of the glide yaml file or some outside config to glide (e.g., an environment variable to a config file aliasing repos)?

I'm thinking of the enterprise use cases where vendoring is used to avoid going out to the Internet regularly to pull dependencies.

Thoughts?