- CLI struct disappeared
- trait Operate to summarize CLI parsing, to allow new subtitle
  operations to be made. Each operation embed all its required
  arguments and functions.
- new module `operate` for operations, extracted from the main and srt
  modules.
- srt module is now just about parsing and dumping srt files. Further
  abstraction could easily be made to allow more file types.
- --from and --to have been implemented on the new Shift struct, that
  implements Operate
- rustdoc