Add support for report filtering

[Jellyfrog]

Adds the ability to prevent reports from being sent.

The motivation behind it that we need a way to dynamically allow/disallow reports from being sent, for example based on if git tree is clean, which can be compute heavy and cannot be done during boot.

Compared to filterExceptionsCallable it can also prevent logs and errors from being sent, and that's why it's in sendReportToApi() instead of shouldSendReport()

Usage is something like this:

Flare::filterReportsUsing(function(Report $report) {
  return false;
});

[AlexVanderbist]

Thanks, another nice addition! I've also documented this method in the Flare docs.