feature request: Generate multiple interfaces into the same file

[OscarVanL]

I want to generate interfaces over the GCP Datastore library. This has four structs: Client, Transaction, Iterator, and Commit.

Ideally I'd like to use interfacer to generate these all into the same target file, perhaps -for and -as could take multiple arguments to a single -o output file.

Unless I am mistaken, I believe this is not currently possible. It would be a nice feature :)