suggest name when starting feature branch and convert spaces to underbars

This changes the start command so that it suggests a branch-name suffix when asking for a branch name.  The suggestion is the name of the story with spaces replaced with underbars.  If the user does not type in a branch name, then the suggested suffix is added to the branch name. If the user does type in a branch name, then the string they typed is used.  If the resulting branch name contains spaces, they are replaced with underbars.