Provide greater flexibility to auto-link JIRA Issues to source control commits

Currently JIRA only links commits based on searching for the JIRA Key (<PROJ-NUM>) in the commit message.

It would be very useful if users could add additional criteria to link Issues and Commits. In particular this is necessary for migrating clients (e.g. from Redmine) to allow historical linkages to be maintained without rewriting your source control history.

Example: Importing from Redmine

• Redmine looks for #<issue_id> to associate commits with issues
• The JIRA/Redmine importer brings the Redmine issue_id into the "External issue id" field by default

If it was possible to add a configuration such that JIRA searched for "#<External issue id>" along with the JIRA key then all historical commit-issue linkages would be maintained.