Reconfigure/disable logging

[blueyed]

While looking at #614 I've noticed that it is not possible to disable or re-configure logging, although that should be possible.

The only problem I see is that e.g. deoplete-jedi uses deoplete's file handler for its server's logging, which would need to be restarted for logging changes.
This could be achieved either by calling "Init" again (not that nice), or having a new event for when logging config changes.

[Shougo]

I don't want to add the feature.
But the PR is wellcome.

It should be implemented after #602.