Command line arg for config file

[malaney]

Firstly, I'm loving gassetic and what it has done for my frontend asset mgmt. Great job! I'd like to request that we add a command line switch (i.e. --config) that would allow one to specify a config file to be used in place of the standard gassetic.yml. gassetic.yml would still remain as the default, but now developers would be able to specify an alternate config (say gassetic_dev.yml) to use that instead. Having this feature would allow us to reduce the amount of environment specific switching that happens in our templates and replace that with environment specific switching in our configs.