Includes: add helper function for linking to PRs/issues

Author: harding

_includes/link-to-issues.md

@@ -0,0 +1,4 @@
+{% assign _issues = include.issues | split: "," %}
+{% for _issue in _issues %}
+[#{{_issue}}]: https://github.com/bitcoin/bitcoin/issues/{{_issue}}
+{% endfor %}