Unable to create multiple instances

[blakeembrey]

It's possible that someone would need to install the module more than once to support different file retrievals. For example, using the babel CLI and it uses this module with in-memory source map look ups. However, someone that wants to add support for the rest of the application now can't. I propose removing the global overrides of retrieveFile, etc. and instead using an array of functions that resolve to the first supported.

[dgreensp]

This seems really important for wider adoption.