added bitpay config command

Adds the command `bitpay config` for managing multiple configurations. This is pretty badass.

View the current config: 
```
bitpay config --print
```

List possible preset configurations: 
```
bitpay config --list
```

Load a preset configuration: 
```
bitpay config --use prod
```

Modify configuration: 
```
bitpay config --set apiHost --value localhost
```

Save your config as a new preset for later use: 
```
bitpay config --save local
```