Fix/positional args with underscore

This should fix issue #122

Positional arguments with underscores are flagged as positional by storing the name *with* the underscore in a list. However, the flag is set somewhere else in the code, where the underscore has been replaced for a dash. Hence, the positional test fails and the task argument is left in a weird limbo of not being neither positional nor optional. Also, the argument was getting the default value for positional arguments (None).