fix hTag matching

Hi folks !
I noticed that the regex used for hTag recognition is too restrictive, and that results in missing anchors. I changed it from `<h([1-6])>.+<\/h\1>` to `<h([1-6])[^>]*>.+<\/h\1>`.

This way it handles cases like `<h1 id="title">some title</h1>`
This should fix https://github.com/maximebf/beautiful-docs/issues/35