Added `disqus:script` and enhancements

Added `{{ disqus:script }}` which only outputs the script of Disqus in
case you want to include it on a page for generating comment counts but
not actual comments.

I also added a check to see if the script has been output on the page
before so we do not output twice accidentally.

Also defaulted the ID and URL to the current `uri_string()` and current
URL for times when you are using this plugin outside of the Pages
module. This will ensure correct comment threads for all pages.
Updated read me based on previous commit
Merge pull request #3 from chadwithuhc/master

Added `disqus:script` and enhancements