As part of working on #391, I realised that typo'd flags in the CLI don't stop processing. Since ktfmt overwrites files by default, that's pretty dangerous!

E.g.

$ ktfmt --dry-runn SomeFile.kt

will report the unexpected option --dry-runn but will format and overwrite SomeFile.kt anyway!

I thought this was enough of a showstopper to report it separately.

IMO, all unknown flags should be errors, and no processing should take place when they're encountered.