Turn Options::Processor into a facade.

So it hides all the internal options processing and we can use just
call this one static method in bin/jsduck.
Refactor the long if-statement in Validator.
Perform the export options check only in Validator.

Previously the logic for this check was duplicated and sadly wrong.
Move --guides-toc-level validation to Validator.
Describe validation rules inside Options::Parser.

Eliminate the Validator class and attatch all the validation rules to
the Options::Record object, on which we can later simply call #validate!
to perform its own validations.