Allowing some messages to always be flushed from FingersCrossedHandler

This PR adds an optional argument to specify a minimum level of message to always flush through when FingersCrossedHandler is closed.

The purpose of FingersCrossedHandler is to prevent your logs from filling up with debug-level junk by only logging it in situations where something has gone wrong. However, I sometimes still want to get a basic level of logging in normal situations, and this is not possible with FCH at the moment; it's either everything or nothing.

It's possible to work around this in the current monolog by using FilterHandlers, but currently that will always cause either duplicate or out-of-order messages (depending on how you arrange the filters).