Add fallback behavior to editor

[JeremiahSecrist]

Currently if the editor is improperly set a user will run into an error and it will fail to run. Most times users have an editor on their system but could feasibly improperly configure the editor. This simply checks if a program exists including the first editor that works as the editor.

[n8henrie]

I worry that this is a little too opinionated; if a user doesn't have EDITOR set, I think failing seems reasonable.

Most nix users will be familiar with EDITOR and how to configure and override its value. Some very novice users may not, but I don't think dropping them into vi (as opposed to nano, which you have lower on the list) is going to go over well! (Cue the "how to exit vim" jokes.)