Add an example of how to parameterize a between predicate from earlier query values

[krlawrence]

An interesting case where 3 by modulators are needed to parameterize a between predicate.

gremlin> g.withSideEffect('a',5).
......1>   withSideEffect('b',8).
......2>   V().values('runways').
......3>   where(between('a','b')).
......4>     by().
......5>     by(math('a + 1')).
......6>     by(math('b - 1'))