[RFC] Added ability to define extra reporters for use in command/controller

As talked about in #69

Extra reporters are defined with a service tag:

```yaml
my_reporter:
    class: My\Reporter
    tags:
        -  { name: liip_monitor.additional_reporter, alias: my_reporter }
```

For the web check version, you can enable the reporter in the query string: `/run?reporters[]=my_reporter` or `/run?reporters=my_reporter`

And for the command: `app/console monitor:health --reporter=my_reporter --reporter=another_reporter`