Allow namespace resolving

Will allow namespaces to be targeted for resolving:

`other@adapter:post` resolves to `other/adapters/post`

This is different than the Custom Type Prefixes in
https://github.com/stefanpenner/ember-resolver/pull/16 as that feature
only allows one chance to access the module. But if you want to have a
default module path and if that fails lookup a similar path in another
prefix the Custom Type Prefixes won't work and you must target the
namespaces specifically in the resolving path.