support ZAW_EDITOR value
support ZAW_EDITOR value

Usually many user use EDITOR as commit message editor, so
EDITOR block process.
Therefore I don't want to block process when editor is opened
from zaw. So I use separate environment value ZAW_EDITOR.
If ZAW_EDITOR value is not set, zaw try to use EDITOR.

g5pw/zaw,junkblocker/zaw,pengjiguang/zaw,willghatch/zaw,termoshtt/zaw,vapniks/zaw,brandon-fryslie/zaw,elek/zaw,xeno1991/zaw