Improve parsing of csslint output

Summary: csslint returns an evidence field which contains the line of the "original text" which produced the linter message (see https://github.com/CSSLint/csslint/blob/5dd84b259b90c5ea8adbaf03d31e9df093cc3dcd/src/core/Reporter.js#L64). We can use this data instead of trying to achieve the same result using $this->getData($path).

Test Plan:

Reviewers: chad, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11311