GitHub Issue 351: Ability to send additional runtime data

[ArturMoczulski]

This PR addresses #351

Added new config option custom_data_method which allows for custom logic to add custom data to a Rollbar request performed on runtime when the error is being reported. Additional context data can be passed to this callable through the $extra or $context parameters of Rollbar::log and RollbarLogger::log under the custom_data_method_context key. This is the same feature as in the Ruby gem SDK https://docs.rollbar.com/docs/ruby#section-including-additional-runtime-data

Example of how to use this feature has been presented here: https://github.com/rollbar/rollbar-php-examples/tree/master/custom-data-method

The docs have been amended.