post- and presync hooks for quick and full syncs

[dopefishh]

The postsynchook and presynchook are configuration options to execute a process before of after synchronisation.

In my case it would be useful to discriminate between quick syncs and full syncs in regards to post- and presynchooks. My postsynchook is quite CPU intensive and I would like to only run it on full syncs and skip it on quick syncs.

To keep it backwards compatible I propose to add four new configuration options so that there are a total of six synchooks:

• presynchook
  Executes before both a quick and a full sync
• postsynchook
  Executes after both a quick and a full sync
• prequicksynchook
  Executes only before a quick sync
• postquicksynchook
  Executes only after a quick sync
• prefullsynchook
  Executes only before a full sync
• postfullsynchook
  Executes only after a full sync

I'm fine with implementing this but I wanted to poll enthusiasm first.

[nicolas33]

I'd merge patches. You might like to contribute to offlineimap3, though.

[nicolas33]

BTW, I think the best way to implement this is not to introduce yet another configuration option. I'd rather add a new "context" argument object when calling the user function.