feat: add feature to pad iteration number

[karrukola]

This change addresses the discussion in issue #87.

[karrukola]

Submitting logic for review, if OK, e.g. usage of f-strings does not cause problems, I would add the tests:

• When padding is given and the number of repetions is less than 10, then no padding is applied
• When padding is set to 0 and the number of repetitions is 10, then the test names are 01, 02...
• When padding is set to " " and the number of repetitions is 10, then the test names are 1, 2...
• When padding is set to "" (emtpy string) and the number of repetitions is 10, then no padding is applied
• When padding is set to two characters, then a warning is emitted and no padding is applied

When running tox, all Python 3.7 jobs fail, it must be the installation on my machine; I get the same results with a clean checkout of main.