Setting it so any links in chat-topic open in new tab

Using jQuery .attr() to set target="_blank" on any links in MediaWikiChat's
chat topic. A similar code had been manually added in enbricki's Common.js,
however this patch should be available for any MediaWikiChat users without the
need for an addition to Common.js.

Change-Id: Ic577aaa9d489a55b14d197c229a2e05f5d483e52