Header offset setting

I'm writing a page which displays articles and consists of a page title as H1, and then the article content translated from markdown.  I'd like to be able to start the header numbering of the content at H2, so I've made this fork.  This is related to https://github.com/michelf/php-markdown/pull/102, but I prefer my implementation.  There is an open issue for this feature here: https://github.com/michelf/php-markdown/issues/117