Add uninstall command

[cbracken]

This would uninstall the specified version with the following behaviour:

1. abort if any aliases exist
2. abort if it's the current use version
3. abort if it's the default use version
4. otherwise, delete the version

[astrolemonade]

I would like to have an uninstall command too! I usually implode dvm and install the latest version

[hotdang-ca]

I'd love to try to implement this.