* Remove modify_hr_tags method definition
this breaks things that call it.
* Remove direct uses of modify_hr_tags fixer method
Remove from class METHODS lists, remove test cases about this behavior, and remove from
methods lists in test cases.
* Remove hr tag modification spec
Still don't understand what problem this was fixing, but I've removed
the test case.
* Create new MarkdownProcessor::Fixer services
* Remove old MarkdownFixer
* Code cleanup
* Capitalize Base in code comments
* Remove comments related to inheritance
* Add fix_methods method to hold METHDOS constant