✨ add different strategies for module synchronization

[jeftadlvw]

The current synchronization behaviour is fixed. If a submodule does not exist at it's set location, the configuration is chosen as the source of truth and the module is cloned. But if the module already exists, the module is set as the source of truth and the configuration is changed.

It should be possible to define and set this behaviour in the configuration with an optional field:

[[submodule]]
  path = "path/to/repository"
  url = "https://url/to/repository"
  sync = "SYNC_STRATEGY"

It should also be a flag when synchronizing single submodules (#9) for setting the strategy temporarily.

Possible strategies could be:

• module: if the module exists, it's the source of truth
• config: set the configuration as the source of truth. An error is displayed if a checkout or url-change would result in lost changes.
• ignore: just create the submodule and ignore all further changes