Export: Count levels relative when doing subtree export.

Export: Count levels relative when doing subtree export.

William Henney writes:

    Consider a structure like this:

    * first
    ** second
    *** third
    **** fourth

    Currently, if I export the "third" sub-tree to html (via "C-c @
    C-x C-e h"), then "third" becomes an <h1> heading, but "fourth"
    becomes an <h5> heading. I would rather it were <h2>, i.e. that
    all heading levels be relative to the root of the sub-tree. Is
    there any way to achieve this?

This was not possible so far, but this commit measures the first
headline of the subtree and applies an offset to all level values.