Option to only add a __repr__ if none is present in the current source

[cleder]

The current behaviour is to generate a __repr__ if it can be generated, regardless if the class currently has a __repr__ or not.

The add command should only generate a __repr__ if the class does not have one,
Introduce an optional switch like --ignore-existing to add a __repr__ regardless.

[cleder]

To clarify:
with --ignore-existing it will add a __repr__ to classes that do not have a __repr__ and skip the classes that do.