Add support for STRIPE_API_KEY in .env

[jeffl8n]

Reviewers

r? @tomer-stripe
cc @stripe/developer-products

Summary

Add support for using a .env file in the current directory that contains the STRIPE_API_KEY key/value.

Addresses #653

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[jeffl8n]

r? @fhats-stripe
r? @tomas-stripe
r? @tomer-stripe