Orgtbl compat

This branch adds additional syntax to the table extension such that the `+` character can be used as a cross-section character in header separators. This is particularly convenient for emacs org-mode users as it has tools based on this syntax. e.g.:

    | First Header | Second Header |
    |--------------+---------------|
    | Content Cell | Content Cell  |
    | Content Cell | Content Cell  |

This addition is backwards compatible and does not affect existing functionality.

Tests (all passing) and docs updates are included.