Test for either asterisks or underscores

Pod::Markdown has switched to double asterisks for bold formatting, and
the tests previously only accepted double underscores, resulting in a
failure. This now test for either asterisks or underscores for both bold
and italic, so the tests should pass regardless of what Pod::Markdown
decides to do in the future.

Fixes #13. See also https://github.com/rwstauner/Pod-Markdown/pull/10
Acknowledge ETHER's contributions in module's POD
v0.141120

    - Adjust tests for change in Pod::Markdown output