Trailing and leading underscores appear in flag names

[bitprophet]

E.g. a task with signature def foo(myarg_=True): will try to be processed as if its flag variant was --myarg- due to the straightforward dash-to-underscore transformation.

These should just have the leading/trailing underscore ignored for purposes of flag transformations, since they're usually added simply to get around Python local-namespace issues and don't make much sense as literal flag names anyway.