Add ability to control how exceptions are grouped for error_grouping

[westonganger]

We should be able to control how the exceptions are being grouped.

Out of the box the errors are grouped by name and message. This is not very unique.

I want to be able to provide a proc to generate custom grouping identifiers

get_group_identifier: ->(exception, env, data){
  [
    exception.name,
    exception.message,
    env.ip,
    exception.backtrace.first,
  ].join("/")
}

Without this feature the error grouping feature seems not very useful for generic use.

[OskarEichler]

Yes this would be great - we enabled error_grouping and it doesn't seem to do anything. Getting 1000s of emails of the same errors within minutes -.-

@Martin91 I know that you originally implemented this - would it be possible to add options for more specific grouping?