create option 'atdcat -exclude-names REGEXP'

This creates an option that makes it possible to filter out certain ATD definitions from a file.

Currently I use this new option together with 'atdcat -i' to inline pieces of records in my ATD specs and then hide the "abstract" records from the final .ml* files.

Example:
```
type _named = { name : string }
type obj1 = { inherit _named; x1 : int } <ocaml field_prefix="o1_">
type obj2 = { inherit _named; x2: string } <ocaml field_prefix="o2_">
```
Here I would typically filter out the '_*' types.

This said, I agree this is a bit sketchy and I could live without this option. (In the end I don't use it so much.)