"Allow" usage of !!.

[nning]

The current (freshly forked) version of the style guide states "Avoid the use of !!.". The bad examples there are for nil checks and on booleans. I would tend to allow it for "casting into booleans to return only boolean" as in...

def display_commit?
  !! Settings.display_head_commit
end

But I would presume, not to use a space between "!!" and the following term to be consistent with the "No space after !." rule.

[0robustus1]

Somewhat part of pull-request #4. Maybe needs further discussion?