Add \underline support (#456)

![image](https://cloud.githubusercontent.com/assets/6820/14412284/e63218b6-ff13-11e5-912c-fc6e30928b93.png)

Fixes #209.

test/screenshotter/ss_data.yaml

@@ -85,7 +85,7 @@ NullDelimiterInteraction: a \bigl. + 2 \quad \left. + a \right)
 OpLimits: |
     {\sin_2^2 \lim_2^2 \int_2^2 \sum_2^2}
     {\displaystyle \lim_2^2 \int_2^2 \intop_2^2 \sum_2^2}
-Overline: \overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{y}}
+OverUnderline: x\underline{x}\underline{\underline{x}}\underline{x_{x_{x_x}}}\underline{x^{x^{x^x}}}\overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{\underline{x}}\underline{\overline{x}}}
 Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x}
 PrimeSpacing: f'+f_2'+f^{f'}
 RlapBug: \frac{\rlap{x}}{2}