Add support for multi-line strings per SE-0168

[jtbandes]

See: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170417/035923.html

(Stole some macro/snippet ideas from Python.tmbundle.)

cc @mkhl @patrickrgaffney

[jtbandes]

Made some additional fixes to handle corner cases such as

_ = """
  hello\("""
     world
     """)
  abc
  """

Tested on: https://github.com/apple/swift/blob/master/test/Parse/multiline_string.swift https://github.com/apple/swift/blob/master/test/Parse/multiline_errors.swift