Added size offset

Hi Stephen,

Great plugin. I am using it for a client project in a design with pretty big font sizes and noticed it gets slow pretty fast. That's why I added some options to decrease the number of loops for bigger fonts.

- sizeOffset: start calculating font sizes from this given size
- sizeIncrement: increment size on each loop with this value, then subtract by 1 till font fits in element.

These changes considerably speed up the plugin for me. Let me know what you think.