Added headings option

[adam-lynch]

This allows for headings to be disabled (and therefore treated like paragraphs). I know you can do it without a config option but it's awkward, I just did it. You need to:

• Manually use the lexer and parser.
• Create your own lexer.
• Override the lexer rule with a noop after it's instantiated.
• Make sure you do the above correctly or else any future instantiations will also have this noop instead of the original rule.
• Make sure you pass your options to the lexer and the parser.
• Make sure your options were merged with marked.defaults because the lexer and parser don't do it.
• Do this for each custom renderer / use case you have. I have three.

With this change, you can just set headings: false and it's done. The option defaults to true. I've added a test for ignoring headings, one for testing headings as is (i.e. headings: true) and I've updated marked.min.js.

Best "head"-related GIF I found:

[chjj]

There might be a better more dynamic way to do this for any rule: see #579