Move resolve_tag_value() into core, add core.file_ids and core._add_database_option
repair: add a rudimentary parse_args and CLI for interactive deduplication.

Note that write operations are explicitly disabled at present --
all you get is a dry-run. I recommend not doing any file deletions yet.

Minimal usage is 'python3 -m tmsoup.repair dupes -c some_command',
where some_command gets fed a list of newline-separated filenames on stdin.

The argparsing is also overly dogmatic ATM -- it simply assumes you have specified
--stdin and --command, and haven't specified --single, --keep-highest, or --stats
Move various file-related functions to tmsoup.file

core currently contains:
 a) utility functions and constants, like get_db_path(), rename(), delete()
 b) tag-related functions, like tag_files(), untag_files()

In future, I intend to move those in category b) into their own submodule.
And possibly those in a) into a 'util' submodule.