_log returns the class instance for method chaining

[okonomiyaki3000]

So that you can do stuff like:

ChromePhp::group('Some Logs')
    ->log('A log message')
    ->log('Another log message')
    ->log('This can go on as long as you like.')
    ->groupEnd();

Which I think is pretty slick.

[okonomiyaki3000]

⚡

[hopeseekr]

We love fluent interfaces! Hurray!

This PR has been merged into my fork @ PHPExpertsInc/chromephp#4!