Allows specifying dump files individually

[philipmat]

• Fixes Allow specifying direct files rather than --export + input dir #112
• Gains explicit --output argument
• can specify either input directory or multiple input files

$ python run.py --help
Usage:
  run.py [--bz2] [--dry-run] [--limit=<lines>] [--debug] [--apicounts] [--output=<dir>] <INPUT_FILE> <INPUT_FILE>...
  run.py [--bz2] [--dry-run] [--limit=<lines>] [--debug] [--apicounts] [--output=<dir>] INPUT_DIR [--export=<entity>]...

Options:
  --bz2                 Compress output files using bz2 compression library.
  --limit=<lines>       Limit export to some number of entities (all otherwise)
  --export=<entity>     Limit export to some entities (repeatable).
                        Entity is one of: artist, label, master, release.
  --debug               Turn on debugging prints
  --apicounts           Check entities counts with Discogs API
  --dry-run             Do not write csv files.
  --output=<dir> Where to write the csv files. Defaults to current dir.