Make tagged template strings solution a bit more efficient.

See #15.

Author: domenic

lib/exercises/tagged_template_strings/solution/solution.js

@@ -1,11 +1,9 @@
 console.log(html`<b>${process.argv[2]} says</b>: "${process.argv[3]}"`);
 
 function html(pieces, ...substitutions) {
-    substitutions = substitutions.map(escape);
-
     var result = pieces[0];
     for (var i = 0; i < substitutions.length; ++i) {
-        result += substitutions[i] + pieces[i + 1];
+        result += escape(substitutions[i]) + pieces[i + 1];
     }
 
     return result;