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 the downloadNewIndexProperties here is pretty unsafe; we are doing the download in a fairly careful way over in preStartRepository https://github.com/grahamar/activator-template-cache/blob/master/templates-cache/src/main/scala/activator/cache/TemplateCacheActor.scala#L175 including avoiding updates when they are disabled, catching exceptions, only downloading if we need to, etc. If we need a name field on the repo itself I think probably TemplateCacheActor should explicitly set it, and if we try to use it before it's set we'd throw an exception explaining the problem.