Add theme option

[unDemian]

Built on top of #196

Summary

Currently the Happychat client is selecting the color theme based on the groups sent via parameters. This PR updates that behavior and now themes will be selected based on a theme parameter.

There are 2 reasons for this change, first of all groups are deprecated and will be replaced by skills soon. Also at least in calypso we have scenarios where a chat could be part of jpop product but we still need the calypso theme to be loaded.

Testing

1. Run npm start
2. Open http://localhost:9000
3. It should load jetpack's color scheme
4. Update the targets/standalone/example.html theme option to either calypso or woo
5. Run npm start again
6. It should change the color scheme to the one selected.

[oandregal]

This should have been addressed by #241