support [test] and [live] groups in ~/.stripecli config file

Hi there!

very nice cli you got here.  This code just makes use of the "groups" support thats built into the ParseConfig gem to allow for predefining both `test` and `live` secret keys in your `~/.stripecli` config file.  stripe-cli will then use the first group, by default, or the group specified on the command line with the `--mode` parameter, like `--mode=live`.  config files with no groups will continue to work as before, however you can also define one like this:
 
![stripecli](https://f.cloud.github.com/assets/1504753/793087/fb3e7084-ebdd-11e2-997b-29eae8c2156b.png)