Allow multi-line strings (common in SET records).

[abargnesi]

A multi-line SET record will result in an incorrect AST. The value() node is expected but an incomplete string() node is returned:

(set [[:@complete, nil]]
  (name []
    (identifier [[:@complete, true]] "Evidence"))
  (string [[:@complete, false]] "this is a multi\n"))

This issue causes OpenBEL/openbel-api#114.