config: Auto-save logic

[dgw]

It's silly that changing the config via PM with Sopel doesn't save automatically. Many settings only take effect after a restart, so changing them and then restarting the bot (with .quit + an init script/upstart service, or the upcoming .restart command, etc.) without remembering to save is just a waste of time.

I can understand that Sopel might have been designed in this case to minimize disk activity, or force the bot owner to confirm their changes with a manual .save command as sort of insurance against typos. But with the constant disk I/O from the loaded-by-default seen module now it doesn't make any sense to try and avoid extra disk activity caused by low-frequency events like config changes.

With most settings that could cause problems only taking effect after a restart, whether the owner is forced to manually save or not they're going to need to go in and manually edit the config either way if they set something wrong and the bot can't reconnect. So I can't think of any reason not to just auto-save.

[dgw]

Let's not do this until #1691 gets solved. Any auto-save function would clobber config file comments silently (until the user opens the file and notices, of course).