Migrate from oof to clap

[marcospb19]

ouch used to use clap, then we switched to oof, our argparsing implementation, looking back I can tell it wasn't the best decision, my core flag detection implementation still has some little flaws and lacks basic features.

Here are some reasons why clap will be beneficial for ouch:

1. Shell completions.
2. Man page generation (with help2man).
3. Adding flags is easier.
4. No need to edit --help manually.
5. Not worrying about ~90% of the argparsing error treatment.
6. Help wraps if width is limited.
7. Battle tested, our implementation was never 100% tested.
8. Automatic typo checking for flags too (we used to check this manually only for subcommands).
9. Deals correctly with the argument - (single hyphen, can be later be interpreted as STDIN and STDOUT to make ouch more versatile).
10. Deals correctly with -- (separator, files that come after can contain hyphens, and are not interpreted as flags).
11. More consistent error messages.

[SpyrosRoum]

Clap version 3 is currently in beta and it brings (At least) a feature that I find very appealing. It includes a copy of structopt.
Is this something you find appealing or do you prefer the classic way clap works?

[marcospb19]

I like it too, if it adds no limitations compared to the "traditional" 2.33.x of doing things, then my only request is for you to use a specific release with something like a "strict version requirement" (rendered).

Here's an example in action with durt: cauebs/durt@94e5b60.

This way we avoid beta updates breaking your clap code (when it downloads a more recent version, but with breaking changes).

[SpyrosRoum]

Agreed, we should keep it pinned while it's in beta