Increment header levels in markdown compiler instead of stripping h1's

Slight change to behaviour of the markdown plugin:
Rather than stripping out all h1 tags this version increments the numbers in all header tags (i.e. `<h1>` becomes `<h2>`, `<h2>` becomes `<h3>` etc.)
This allows markdown documents to use `=====` underlined headers while still reserving h1 for the title.