Convert issue numbers in the changelog into links

[nicwortel]

Issue numbers in pull request titles should be converted into links, linking to the issue tracker URL for that issue.

This should support:

• JIRA issue numbers (ABC-123) where "ABC" should be a configurable prefix
• GitHub issue numbers (#123) - make sure that this differentiates issue numbers from pull request numbers

Perhaps the user should be able to configure a regular expression for detecting issue numbers.

Requires #4