Add an example of re-using a value calculated earlier in a traversal

[krlawrence]

Need to add an example along these lines

gremlin> g.V(50,8).values('city').as('c').V().filter(values('desc').where(TextP.containing('c'))).values('desc')
==>London Heathrow
==>London Gatwick
==>London City Airport
==>London Stansted Airport
==>London Luton Airport
==>London Southend Airport
==>London Airport
==>Dallas/Fort Worth International Airport
==>Dallas Love Field