Added custom header support

After using marked for awhile and reviewing the multiple pull requests about how to render headers, I thought it would be nice to have an extensible interface to customize header markup.

As a user, I personally want something like GitHub headers where a relative link appears when the user hovers, but I also understand that some people want simpler headers with just an anchor tag or the current one with just an id.

To keep everyone happy, I thought we could expose the header rendering as an option so that some users can simply use the default rendering while others can do cool GitHub headers or other creative stuff.