added bitpay config command

[tacticalchihuahua]

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

[tacticalchihuahua]

maybe @pnagurny wants to review and test?