Make option method public

This includes many changes, some of which are not backwards compatible.

- option now takes a list of argNames, which is primarily used for determining
  how many arguments an option takes (now a fixed number), but is also used in
  help and error messages.
- added a bunch of tests, and fixed bugs that were discovered with their
  creation, particularly around positionals and default/addValidator

Also moved each test to its own class. This seems to play better with
IntelliJ:
- faster editing and refactoring
- can run individual tests

Author: xenomachina

CHANGELOG.md

@@ -7,8 +7,20 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Added
+
+- `ArgParser.option` is now a public method, so it's possible to create many
+  new option types that were not previously possible. The existing option types
+  are all written in terms of `option`, so they can be used to get an idea of
+  how it works.
+
 ### Changed
 
+- The `adding` and `positionalList` methods of `ArgParser` have had their
+  parameters slightly reordered to be consistent with the other methods. The
+  name(s) come first, if any, followed by `help`.
+
+
 - Made Delegate and DelegateProvider into abstract classes with internal
   constructors. This makes it much easier for me to separate internal and
   public parts of their API. THIS IS AN INCOMPATIBLE CHANGE, however it