Allow non-npm urls

[DylanPiercey]

npm install jashkenas/backbone

It would be handy to use github url's with the cdn, similar to npm.

[DylanPiercey]

If this gets added; Scoped modules should also be supported.

npm install @namespace/name

[jfhbrook]

Because extra slashes are already used to specify subfile requires, we can't adopt this exactly as npm exposes it.

Possible there's another way to do so, maybe with arbitrary git+ssh urls.

[jfhbrook]

More discussion: https://github.com/jfhbrook/wzrd.in/issues/105

[jfhbrook]

Note to self: This would go into the "version" field, so module@git+ssh://.....

[jfhbrook]

Might be mutally exclusive with #57.