Fix a lexer issue with SimpleOptions and backslashes in unterminated quotes

Summary: The SimpleOptions lexer has an error when parsing a line like `a="\`, where the lexer can't accept the backslash inside the unterminated quotes.

Test Plan: Added failing unit tests; they now pass.

Reviewers: btrahan, SEJeff

Reviewed By: SEJeff

CC: SEJeff, aran

Differential Revision: https://secure.phabricator.com/D7323