Init with version number

Hi, 

It can sometimes be useful to init to a specific version, for instance when inserting flyway in an existing project with several production and development databases. 

The idea here is simply to be able to chose which version to init to. 

It's only a draft the UI part is not really satisfying, we are open to discussions to improve that (we're not that comfortable with how to actually submit a form or something similar). 

This patch has been tested and work on one of our projects.