RFC-compliant TXT validation

[jcnelson]

The parser only accepts TXT records with a single double-quoted string, with no escaped quotes. For example, "foo" is fine, but "foo" "bar" is not, and "foo \" bar" is not. However, these last two examples are RFC-compliant. The parser should follow the RFC as best as it can. Details here: https://tools.ietf.org/html/rfc1464