Stop Disqus "Comemnts?" link with no username

The twitter() method wasn't checking for an empty string for
global_disqus_username, so it would always generate the anchor tag for
the Disqus comments, even if global_disqus_username was an empty
string.  Now the '-z' flag has been added to check for the empty string
correctly.
Merge pull request #20 from Datyedyeguy/master

Stop Disqus "Comemnts?" link with no username