Add support for EDITOR environment variable

[HeroicKatora]

This configuration will overwrite the default one while still being overwritten by an explicit command line argument. This handling matches the rules found in other programs such as git etc.

As noted in this comment similar projects also use this environment variable.

You're missing an explicit license but I'm okay with donating this code under any open source one, regard it as being in public domain as much as possible. If you have any trouble choosing one, I have some experience.