Basic config support:

Hi, Nathan,

I've taken a stab at adding basic configuration support to green. Let me know what you think.

 - green                        # load config from $HOME/.green if it exists
 - export GREEN_CONF=zzz; green # load config from file zzz (override $HOME/.green)
 - green -c xxx                 # load config from file xxx (override $GREEN_CONF)

Config file syntax is defined by python's ConfigParser.