implement parser for multi-line string literal (a.k.a. heredoc); syntax is borrowed from python
disable unnecessary captures (amends prev. commit)
emit multi-line string literal with escape
add tests
do not crash while trying to parse an unterminated multi-line string literal
add failing test
Util.decodeStringLiteral should take care of heredoc
Merge pull request #240 from jsx/kazuho/experiments/heredoc

add support for multi-line string literal