add zaw-git-status into readme
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 remote-tracking branch 'byplayer/zaw-editor'
Merge remote-tracking branch 'byplayer/git-add'
revert bindkey to default