Bigtable sources do not work with local emulator

[NarekA]

In order to get the bigtable sources working with the local emulator there needs to be a way to set the host and port in the bigtable options which the current sources do not support.

[pelletier]

See the BigtableIO documentation. I think we should just accept an arbitrarily constructed BigtableOptions$Builder.

[NarekA]

Ya, I was thinking that might be the best way to do it. As much as I like using a hashmap I think it's best to let users make their own BigtableOptions object.