Add Env.is() method

[marcuspoehls]

The environment utility has convenience methods like isProduction() and isTesting(). We can add an is() method to be more flexible in determining the environment an application is running in.

Usage

Env.is('local')
// returns true when NODE_ENV === 'local'

Add the method to the end of the Env utility.

Thank you for your help :)

[marcuspoehls]

Closed by #34