Add submodule support in hotfix, feature scripts

Also added the `feature clean` command to remove untracted files and
leftover submodules.
Remove --dry and --all flags on feature switch
Remove --all and --dry from feature switch help
Merge submodule-support (#36) into master

Add submodule support in hotfix, feature scripts

Also added the `feature clean` command to remove untracted files and leftover submodules.