Introduce an option to configure how unavailable issues are handled

[brianoliver]

When migrating issues from JIRA, sometimes there are "holes" in the issue number ranges, typically caused by issues being deleted from JIRA (instead of just closed). To deal with said "holes", JIM will automatically create "unavailable" (aka: dumby) issues to ensure that all migrated issue numbers are maintained in sequence. However it's possible that this behavior is undesirable, especially if there are a lot of said "holes".

We should provide an option to allow automatic creation of "unavailable" issues to be turned off and when done so, we should output a mapping table highlighting the old -> new issue numbers produced due to migration.