auto add HR for rst source

I found that in `samples/example3/slides.rst`, there is one `hr` before every `header`, 

so we can add it automatic, which make the source file much easy to read 

see `samples/example3/slides_without_hr.rst` as a example, it Compatible the source with `hr`

    Middle Title Slide
    ==================

    Here we Go Again
    ----------------

will change to

    ----

    Middle Title Slide
    ==================

    ----

    Here we Go Again
    ----------------