"Strict" flag to override error in production environment

[dkalamar]

Fixes #10

I have a similar use-case to the issue mentioned in #10 and in an effort to reduce additional dependences, such as env-cmd, opted to alter this code to provide a "non-strict" option to production builds.
I've added a boolean flag "--strict" or "-s" to allow production builds to use the default values provided in the .env file.

I believe this adds two key uses:

• The option to default to existing values in cases where that variable may not always be provided
• The ability to specify environmental variable overrides of the .env for local runs, and observe the effect in a development build.

I also added in a test for a line in production builds that was previously uncovered.

[seanblonien]

@dkalamar could you also add brief documentation describing the flag to the README's "CLI options" section?