Ability to disable reCAPTCHA for testing

Love the package, makes reCAPTCHA crazy easy to use.

I have some Codeception tests that fill out forms (to simulate a user filling out the registration form, etc.) that obviously fail when I add reCAPTCHA, 

I added an 'enabled' config so I could disable it in the config, and a `testing` environment configuration to disable it by default for tests.