Add option to suppress sending email

[sparkchaser]

I recommend adding a configuration option that causes the plugin to create cross-references without sending email updates about it.

In our case, we have a large number of tickets that were created before we installed the subtickets plugin (we were using a much less user-friendly method for organizing subtasks). We are in the process of modifying our existing tickets to specify parent/child relationships. Whenever we mark a ticket as a child ticket, we will end up getting two emails about it: one referencing the change made to the child and another referencing the automatic cross-reference that the plugin made in the parent ticket. The end result of this is a large amount of email traffic to all involved.

It would be extremely helpful to have a configuration option capable of controlling how this plugin sends email updates. By default, it would behave as it does now (send email on all cross-references). Enabling the option would result in never sending email regarding cross-references.