GCC colourization fix and conf files for `mount' and `df'

Hello,
 I've tried to contact the original author but unfortunately I did not get any answer. :(

Since yours is the main repo on GitHub I'd like to hear your opinion on pulling the following fixes and improvements:
* ```mount``` colourization
* ```df``` colourization
* fix ```gcc``` colourization of the ```-o``` option from spilling to the whole line

Let me know what you think about them.

Thanks!