Rewrite formatting checks

[TimJentzsch]

We need to a major rewrite of the formatting check system, especially for header and footer checks.
Instead of checking every rule every time we need a more finer grained approach in order to provide more useful diagnostics. For example, the header check should see if it can be found, then check if it's at the start, then check if the separator is present, etc.

[seeroflights]

To help with this, here's a list of everything I've seen/complained about re: formatting checks. I recognize that these are just symptoms of how the current formatting checks are structured, but they're problems that the bot should've been catching.

• Formatting checks should not apply to code blocks. e.g. writing r/length in code is valid, and shouldn't cause you to be caught by the bot. (Disable formatting checks in code #324)
• Formatting checks should apply to quote blocks. e.g. you shouldn't be able to write:

  > hello  
  > this is a new line,  
  > after two spaces
  

  I know we talked about this in a thread in the QA channel, but I still think it shouldn't be allowed to happen. People who are familiar with transcribing know that they shouldn't do this. IMO this is like the hashtag check. Using a hashtag without a space for the title is still correct in terms of Markdown, but so many people screw it up that it's just better to force everyone to put a space instead.
• The bot only looks for the existence of at least two separators so the Comic format, or the Tumblr format, or really anything where you have multiple separators, doesn't check whether there's a separator before the footer. example
• It should be ok to have spaces on the line before the separator. example
• There should also be nothing after the header before the first separator e.g.

  *Image Transcription: Comic*
  
  [*Name of comic*]
  
  ---
  

• Header italics should work (Detect broken italics in header #341)