Add a `rollback` command

[sgrif]

This command should mirror the behavior of heroku releases:rollback, which optionally takes a single release name as an argument.

• The command should pipe the output of any processes it subshells out to
• Optional nice to have: Some form of confirm dialog
• Optional nice to have: Error if the command is run too frequently -- if we want to go back 3 releases we would specify it by name.

[nellshamrell]

Implemented with #23 (without the optional nice to haves - but I'm happy to come back to those in the future)