Update markdown reference to external issues

1. Issue may be not only in jira.
2. Rewrite method for support different external issue trackers

lib/gitlab/markdown.rb

@@ -189,8 +189,12 @@ module Gitlab
 
           link_to("##{identifier}", url, options)
         end
-      elsif project.issues_tracker == 'jira'
-        reference_jira_issue(identifier, project)
+      else
+        config = Gitlab.config
+        external_issue_tracker = config.issues_tracker[project.issues_tracker]
+        if external_issue_tracker.present?
+          reference_external_issue(identifier, external_issue_tracker, project)
+        end
       end
     end
 
@@ -226,9 +230,9 @@ module Gitlab
       end
     end
 
-    def reference_jira_issue(identifier, project = @project)
+    def reference_external_issue(identifier, issue_tracker, project = @project)
       url = url_for_issue(identifier)
-      title = Gitlab.config.issues_tracker[project.issues_tracker]["title"]
+      title = issue_tracker['title']
 
       options = html_options.merge(
         title: "Issue in #{title}",