Support for multiple template repositories

A working first stab at support for multiple template repositories. Main aim is to allow for internal company repositories so that company specific templates needn't pollute the public repository.
Ah, so the actually index itself has to have the name embedded.   I was curious how you planned to prevent mismatched names (a common issue in ivy).  I like that it's centralized like this and the URL defines things.  Even allows cloning/proxying the repository.