Added option to adjust input width instead of font size

I used this script on a project where I wanted to adjust the width of an inline input instead of the font size. You could technically do this by setting the size attribute of the input based on the number of characters, but in my experience the size wasn't always consistent, so this gets the true pixel width of the input's value. I thought this would be a nice addition to this plugin, since it uses the same cloning technique.

I also updated the documentation and demo.