Implemtent the multi-line string method

[GreasySlug]

Fixes #167.

Add method for multi-line strings and a file to test it.

A test for tokenizing multi-line strings will also be added.

Changes proposed in this PR:

• Add lex_multi_line_str() method
• Add multi-line string and string conditional branching
• Add test to ensure that the added methods function properly

@mtshiba

[GreasySlug]

I found a new bug in implementing this. Tokenized for quotes only, but trimmed after execution.

>>> a = """\"\"\""""

>>> print! a

None

It probably is a bug, since the tokenization test passed.

[mtshiba]

Thank you! We will deal with the bug in another issue.