Store mailchimp variables in environment

Hello @quartzmo,

It is a better practice to store sensitive information like API keys in the environment rather than the code. Here you encourage people to store in git these info.

This PR introduces the `ENV` for storing `MAILCHIMP_API_KEY` and `MAILCHIMP_LIST_NAME`.

(On a related note, you may tell people to fork your project instead of cloning your main repo, don't you think?)

Thanks for putting together this landing page boilerplate!