canceling a command in a sequence will now stop the subsequent commands from being run

Hi Pazz,

I recently discovered alot and I am very happy with it !

However I had a little problem with a binding I use to quickly tag and archive my mails:
     l = prompt 'tag '; untag inbox

If I change my mind after having typed 'l' and type 'escape', the inbox tag is still removed from the message because "escape" will only cancel the current command of a sequence.

I think it would be better if canceling a command belonging to a sequence would at least prevent the subsequent ones from being run (and maybe unapply the previous one but this may be subject to debate).

The following patch is an attempt to do it.

I am not sure it really fits in the logic of the code so I am waiting for your feedback.

Thanks for your work on alot.

Yann