minor change to fix issue preventing passing in an interable for showindex

Added check that showindex_is_a_str before comparing showindex == "default", which causes an error when showindex is an iterable.

It would be nice if the vertical line separating the index from the table could have the same format as the horizontal line separating the headers from the table. Would someone be able to implement that?

I do not understand why there is a failing check -- I have barely changed anything. Assistance would be appreciated.

The failing test on Appveyor is not related to this PR.

Still before I can accept this PR I need unit tests (test/test_input.py, test/test_output.py) and a reference to an issue it tries to solve.