Improve sizing of text underline

I noticed that the underline sizing for font sizes at or below 10 seemed a bit strong. This attempts to balance it out by allowing it to scale with the font size beyond the 10 and 20 thresholds.

![underline size comparison](https://cloud.githubusercontent.com/assets/604125/2924548/980f213e-d737-11e3-8998-0a8f7ab5907a.png)