Allow strings with backslash characters

The regex matching strings don't accept backslashes. Places to update this include:

Grammar specification:
https://github.com/ContinuumIO/datashape/blob/master/docs/source/grammar.rst#the-datashape-grammar

Lexer:
https://github.com/ContinuumIO/datashape/blob/master/datashape/lexer.py#L41

Tests

Start with the string grammar in docs/source/grammar.rst and the lexer rule around datashape/lexer.py:41. Check the existing string-matching tests, then add coverage for strings containing backslash characters. Done means the grammar, lexer, and tests consistently accept those strings.

Written by the indexing model from the issue text.