add UniMath.hs

Besides the ```\mathbf{\infty}``` problem, I think the other type problem of ```\mathbf{\alpha}``` happens more frequently.  

I think we may use the TeX package [unicode-math](https://github.com/wspr/unicode-math) which provides a comprehensive coverage of the mathematical alphanumeric symbols in Unicode standard. 

For example, ```\mathbf{\alpha}``` cannot be process by TeXMath, we may use the unicode-math latex command ```\mbfitalpha``` instead.  The function 'ltxCmdsToUnis' can convert it to the corresponding Unicode character "\x1D736" ( Github does not allow Unicode character above 0xffff, therefore I can only post a Unicode glyph hex number here). 

The 'ltxCmdsToUnis' function can be applied before applying the 'ToUnicode' function. 

I think this approach can be used as an alternative partial workaround to issue [#13](https://github.com/jgm/texmath/issues/13).