Rename to .env

Let's just use an actual .env file instead of creating a new file.

Closes #942

Warn Checklist

• [x] This changes the interface and requires a Major/Minor version change.
• [ ] I have :tophat:'d these changes by using the commands I changed by hand.
• [ ] I have added a dependancy to the project.
• [x] I have considered any potential impact on node-themekit