[0.12][WIP] Block Strings

[danez]

This RFC adds a new form of StringValue, 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.

Ref: graphql/graphql-js#926
ref: graphql/graphql-js@f9e67c4

[coveralls]

Coverage decreased (-0.08%) to 92.757% when pulling f475cdf on danez:012-block-strings into 94525c0 on webonyx:master.