DSL: consider different cypher versions--how to support?

[freeeve]

I'm ok with starting with support for 2.1, but there may be backwards incompatible changes in 2.2, for example. We'd probably need a way to specify version in the parser/language, and allow users of the library to use the one they want. Alternatively we can just always support the latest--and hope that it won't change too much.