use --show-capture as filter

[swbchangle]

use --show-capture the same way as in _pytest.terminal module.

This is related to issue #171. --show-capture=no|stdout|stderr|log|all can be used to control which captured log goes into html file. --show-capture=no disables all captured logs. For failed tests, longrepr is always included in html, even if --show-capture=no