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.
I think you could have been right the first time, looking at the [activator project file UICacheHelper](https://github.com/typesafehub/activator/blob/52012321b3a5a9f9dcf53582664e385d92763718/ui-common/src/main/scala/activator/UICacheHelper.scala#L31)... Although I'm not sure I fully understand the functionality around the seed repository... I'm not sure how it should be handled really, any ideas? Or is it alright?