added a fix for issue #186

Fixed it by adding an optional second backtick.

The only problem might be code runs containing a backtick as last character. I hope that it's sufficient to tell in such a case to add an additional space.

So this example:

    ``dir=`ls -al```

needs to be written as

    ``dir=`ls -al` ``

in order to be displayed (almost) correctly. We could change L32 to read:

    var reEnd = new RegExp(" ?" + this.match[1], "mg");

in which case one blank preceeding the terminating `` would silently be "eaten".

But I don't know whether this seems appropriate.