[REQUEST] new method `add_section()` to table class

[okken]

Related to #2477

Currently there exists end_section=True option for add_row().
But it would be cool to just have a no argument add_section() function to call instead of add_row().
Could be add_divider() or whatever you want, really.

This function should be a no-op if called before any add_row() or at the end with no further add_row() calls.