Add support for custom option formats

[regadas]

Hi @alexarchambault I still need to fix docs etc but wanted to know you think about this.
Also not sure about the naming ¯_(ツ)_/¯

Related to #181

[alexarchambault]

Thanks a lot @regadas!

That looks fine overall, I'll review in more detail tomorrow or later during the week at the latest.

[alexarchambault]

OptionFormatter is fine by me as a name.

[alexarchambault]

Thanks for that! I added some comments.

Also, if you want this to work when extending CaseApp, that is when using case-app like

object MyApp extends CaseApp[MyOptions] {
  def run(options: MyOptions, args: RemainingArgs): Unit = ???
}

you need some more changes.

#176 illustrates what needs to be added for the extra field in Parser (here, defaultNameFormatter) to be propagated through all Parser instances (likely, some override def defaultNameFormatter = underlying.defaultNameFormatter here or there).

At the very end of the changes of #176, it also adds a test case for that kind of use.

[regadas]

Hi @alexarchambault thanks for the review! (Sorry about the re-format some of them still went through).

Yes supporting the case when extending CaseApp will be good 👍

[alexarchambault]

Thanks! I added one last comment.

[regadas]

@alexarchambault Thanks for taking a look at this! 👍

[regadas]

@alexarchambault btw would you consider adding scalafmt? 😄

[alexarchambault]

It could be added. Especially now that scalameta/scalafmt#1362 should be fixed, it may possible not to add it while not messing too much with the current manual formatting.

[alexarchambault]

So merging this, thanks again!