Have task fail if threshold not met

[tkadlec]

Digging the task! I would love to see it fail if the threshold parameter is not met, though. It would be great for enforcing certain criteria are met in order for the project to build.

Example:

        yslow_test: {
            options: {
            info: "grade",
            format: "json",
            urls: ['http://google.com'],
            reports: ['test/reports/yslow.json'],
            threshold: 90
        },

If the YSlow score wasn't above 90, the task would log an error and the build would fail. This could get very helpful when thresholds were set for specific tests using JSON objects as described here: http://yslow.org/phantomjs/.