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.
Merge pull request #28 from byplayer/zaw-editor

support ZAW_EDITOR value