Assert an increase/decrease by an amount (delta) #339
Description
As per #333 and #330, we should be able to assert on the delta of a .changed/.increased/.decreased.
So far (in #333) the proposed syntaxes are:
foo.should.change('bar').by(5);
foo.should.increase('bar').by(5);
foo.should.decrease('bar').by(5);or
foo.should.change('bar', 5);
foo.should.increase('bar', 5);
foo.should.decrease('bar', 5);