Add support for .env files

[jasikpark]

What kind of issue is this?

• Feature request / PR proposal
• Bug report
• Question

Is this issue for the CLI, the Node.js API, or both?
CLI
What is the current behaviour?

specifying the SPEEDCURVE_API_KEY in a .env in the local directory does not provide the api key to the speedcurve cli

What is the expected behaviour?

specifying the SPEEDCURVE_API_KEY in a .env in the local directory does provide the api key to the speedcurve cli

Which version is affected by this issue? Did this work in previous versions?

2.0.8

[jasikpark]

I'd rather not specify the api key directly in the shell, and I'd like to keep the api key associated with the site it's used in