Individual sync_back configuration option

It's now possible to (de)activate Maildir sync back per Maildir source, using
the sync_back source configuration flag. For backwards compatibility with the
previous implementation, the global :sync_back_to_maildir still exists and
must be set to allow sync back. Additionally, sync back is _active_ by
default per source, i.e. unless sync_back is explicitly set to false, sync
back will occur.

Fixes #141
Track change in syncback flag per source, warn user

Check per source if the sync_back flag changed from false to true. If so,
warn the user. sup-sync-back-maildir now stores the URIs of all sources that
have been synced by it in the file sync-back-ok, which we already write as a
status flag.

We need an extra verification step during startup, since index and sources
need to be initialized. It might be better to load the sources separately,
though, just to keep things nicely separated.
Merge pull request #141 from ericweikl/fix-141-configurable-syncback

Maildir-syncback: Individual sync_back configuration option