Add a ThresholdHandler

This handler can be used to not let any messages from a certain lever through, unless they cross a configured threshold.
This is useful together with, for example, a MailHandler when some batch processing is done and you're only interested in
major failure and not a minor one.

doc/02-handlers-formatters-processors.md

@@ -125,6 +125,10 @@
   has accessors to read out the information.
 - [_HandlerWrapper_](../src/Monolog/Handler/HandlerWrapper.php): A simple handler wrapper you can inherit from to create
  your own wrappers easily.
+- [_ThresholdHandler_](../src/Monolog/Handler/ThresholdHandler.php): This handler will buffer all the log messages it
+  receives, up until a configured threshold is reached, after it will pass all log messages to the wrapped handler.
+  Useful for applying in bath processing when you're only interested in significant failures instead of minor, single
+  erroneous events. 
 
 ## Formatters