Small change to ensure styling is only applied to logs with strings

[mattdesl]

After my last PR, #244, there is a slight bug when you do something like this (passing numbers or non-strings) –

console.log(mouseX, mouseY);

This PR should fix that to ensure that styling is only applied when the first two arguments are strings, like this:

console.log('%cMouse:', 'background: red', mouseX, mouseY);