Allow users to set a custom editor env variable

[kelsin]

Many programs allow a custom editor variable to override EDITOR. Now users can
set that on osenv with the following code:

var osenv = require('osenv')
osenv.setCustomEditorVariable('MY_EDITOR')

var editor = osenv.editor()

If the MY_EDITOR environment variable is set, it will be used. Otherwise we
follow the same rules that osenv has used before.

One confusing point is that if you set the custom editor variable after calling
.editor() the previous result will be cached and this will have no effect.

[zkat]

Do you have a particular need or use-case for this?

In general, osenv is meant to be a very small abstraction layer over cross-platform environment stuff, and features like these don't seem like they would belong with that stated purpose.