Swift4 multiline support

[kenji21]

Tailor Version: v0.1.0 master

Swift Version: 1.0 4.0

Platform (Mac/Linux/Windows/CI): Mac Mac

Installation Method: Homebrew

Steps to Reproduce Issue

1. Add some multiline string vars to StringAndCharacters.swift test file:

let multilineString = """
multiline
text
"""

let multilineStringWithVar = """
multiline
text
with a star \(star)
"""

let multilineStringWithEscaped = """
  You could write multi-lined strings
  without "escaping" single quotes.
"""

Expected Behavior

No test failure

Actual Behavior

java.lang.AssertionError: Expected: an empty string but: was "line 8:24 extraneous input '"' expecting {<EOF>, ';', 'for', 'case', ......

when running GrammarTest::testGrammar