[shell] auto-close backticks

mode/shell/shell.js

@@ -129,6 +129,7 @@ CodeMirror.defineMode('shell', function() {
     token: function(stream, state) {
       return tokenize(stream, state);
     },
+    closeBrackets: "()[]{}''\"\"``",
     lineComment: '#',
     fold: "brace"
   };