Implement the CLI facade

[rr-]

Right now everything we've done may be covered by unit tests which is nice, but it hasn't been used in any way in main.cc. We should finally make a working facade that does something, and test it, ideally w/o spawning a new process (because it's fragile).

[rr-]

Added some preliminary implementation in d48573c, but it still requires two things

1. To be testable
2. To tell errors when the user tries to --edit (and similar) the stream in read only mode (e.g. without providing -o nor --in-place)