Added exclude option to custom config; updated documentation

[smcelhinney]

Added an extra property for configuring a blacklist (environment properties to remove from the configuration). Useful in the context of Google Cloud Functions where NODE_ENV is not an allowable environment variable

[colynb]

@hubspotdev @JakeChampion @smcelhinney Thanks for this PR. I haven't been a great steward of this project because I've been very nervous about the potential of breaking changes. There are a lot of developers who depend on this in ways I can't possibly imagine and so any change, even small and seemingly helpful could have negative impacts on people. So I tend to ignore these instead of responding. Sorry about that. With that said, looking at this, it doesn't appear that it could be a breaking change. Give me a few more mins to review this.

[gasp]

This also solves this issue: when you are storing AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY (in local env) into your .env those env vars are bubbling up to the lambda function and causing this error.

An error occurred: HelloLambdaFunction - Lambda was unable to configure your environment variables because the environment variables you have provided contains reserved keys that are currently not supported for modification. Reserved keys used in this request: AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY (Service: AWSLambdaInternal; Status Code: 400; Error Code: InvalidParameterValueException; Request ID: 1234567a-1234-1234-1234-aa1111111aa1).

I am putting the full error message so that it will be indexed by search engines and avoid headaches to a lot of people.