Add "rule" attribute to "lintxml" output format

Summary: Currently, there's no way to determine which rule generated a lint
message if you have a parser on top of CSSLint. Provide a `rule` attribute
which has the rule ID.

(The specific motivating use case is that I have a program which runs multiple
linters, manages lint message severity above the linter level, and has more
severity levels than CSSLint itself does, so using flags like `--ignore` would
require special casing and reduce flexibility.)

Test Plan: Added new test coverage and executed unit tests.