Made issue linking more flexible

This PR makes the issue linking more flexible. Instead of hard coding the url for JIRA issues, it is now possible to link them to any issue system out there (jira, redmin, trac, ...).

For this case I added a new field where you can define the replace pattern. 

Lets say we have the following matching pattern `refs #(\d+)` with the issue system url `http://redmine.my-project.org/issues/`, then you would use the replace pattern `$1` so your issue URL will look like this: `http://redmine.my-project.org/issues/12345`.