Add filename to the indexer settings as 'command_line.filename'

I've changed the signature of `get_input_io` to take the indexer, at least for now just so you can store the original filename in the settings. This allows a savvy reader to query the filename to make good guesses (e.g., look for .gz to provide gzip decompression, look for .xml to use marc-xml parser, etc.)

Added a command_line test file that does nothing but verify this is set for now, but we can add more to it later.

Should affect no existing code unless you're digging in and calling `#get_input_io` manually.