Fix boolean invert helptext (issue #65 )

When looking at an invoke task's helptext with a flag that defaults to true (engaging the inverse_flags machinery), no indication of the --no-flag syntax is given.  This is confusing, because true-defaulted and false-defaulted flags look the same in the helptext, but have different behavior (no automatic --no-flag for false-defaulted flags).

This patch describes the additional inverse flag in the helptext.