Make concurrent under specific conditions

- Use a wait group to measure when entire task is complete
- For two cases, independent files and files as episodes in lexical
  order use 2 go routines to parse output of HandbrakeCLI
- Turn off concurrency when in file/chapter mode because we can't
  meaningfully process all files until we have analyzed chapters
- use a channel to limit simultaneous go routines
- move episode increment logic outside of go routine and copy current
  value into the closure