Programmatically setting environment

Sometimes maybe useful to set the environment programmatically, instead of relying on  `NODE_ENV` it can be set using the API.

The API can add another function which set an environment according to any given string. An alias for common operations can be useful as well. Example:

```javascript
env.set("development")
env.setDev()
env.set("prod")
env.setProd()
````