Add `timeline` & `timelineEnd` in `plugins.js`

In Chrome and Opera the console object now includes two new methods:

 * `timeline`
    https://developer.chrome.com/devtools/docs/console-api#consoletimelinelabel

 * `timelineEnd`
    https://developer.chrome.com/devtools/docs/console-api#consoletimestamplabel

Close: h5bp/html5-boilerplate#1559.