Added a '--subpixel' option to freetype.rb

I added an option to freetype.rb to enable subpixel rendering. I noticed that by default wine would show jagged fonts for smaller font sizes which was related to FT_CONFIG_OPTION_SUBPIXEL_RENDERING not being defined in ftoption.h.

![screen shot 2014-03-26 at 13 09 53](https://f.cloud.github.com/assets/172588/2524354/3de79110-b4e0-11e3-9b2c-69f37db43ab0.png)
![screen shot 2014-03-26 at 13 08 55](https://f.cloud.github.com/assets/172588/2524355/3de8f122-b4e0-11e3-99e2-f0e232ac0fc4.png)