Add support for WhatFailureGroupHandler (Monolog ~1.11)

Please tag a new version after merging this :-).

Here is an example configuration that can be used to send messages into a Logstash socket.
That socket might not exist or be not writeable in case Logstash crashes/does not run,
but you don't want that to affect your application.

```yaml
monolog:
  handlers:

    ignore_failures:
      type: whatfailuregroup
      members: [ main ]

    main:
      type:  socket
      connection_string: unix:///path/to/logstash/socket
      level: debug
      connection_timeout: 0.05
```