Add backend filter fold option to multi_backend

Adds an option that can be passed to multi_backend bucket folds. The
option {backend, [BACKEND_LIST]} will cause the fold to skip over any
backends not explicitly named in the backend list.

This enhancement addresses issue #1036.

This is my first Basho PR, so apologies in advance if I've neglected any
important details! Fingers crossed :)
Wow, I didn't realize this assumed atom for the BackendName. In the [configs](https://github.com/basho/riak_kv/blob/develop/priv/multi_backend.schema#L59) they are all binaries so they can be set via bucket props.