This RFC adds a new lexed token, the multi-line string, similar to that found in Python and Scala.

A multi-line string starts and ends with a triple-quote:

```
"""This is a triple-quoted string
and it can contain multiple lines"""
```

Multi-line strings are useful for typing literal bodies of text where new lines should be interpretted literally. In fact, the only escape sequence used is `\"""` and `\` is otherwise allowed unescaped. This is beneficial when writing documentation within strings which may reference the back-slash often:

```
"""
In a multi-line string \n and C:\\ are unescaped.
"""
```

The primary value of multi-line strings are to write long-form input directly in query text, in tools like GraphiQL, and as a prerequisite to another pending RFC to allow docstring style documentation in the Schema Definition Language.