Add Ignored Attributes Option to Reap

This PR does two things:

1) Adds an ignored attributes option to reap.
2) Renames the `env` option to `target_env`
Could you do this without the block? Seems like it adds a level of complexity for little benefit. Mapping `(&:strip!)` will modify them in place so you could do `@ignored_attrs = parsed_ignored_attrs_from(input)`.