Support `{{unless}}` in virtual dom templates

lib/javascripts/widget-hbs-compiler.js.es6

@@ -140,9 +140,13 @@ class Compiler {
         }
         break;
       case "BlockStatement":
+        let negate = '';
+
         switch(node.path.original) {
+          case 'unless':
+            negate = '!';
           case 'if':
-            instructions.push(`if (${node.params[0].original}) {`);
+            instructions.push(`if (${negate}${node.params[0].original}) {`);
             node.program.body.forEach(child => {
               instructions = instructions.concat(this.processNode(parentAcc, child));
             });

script/test_hbs_compiler.rb

@@ -5,6 +5,9 @@ template = <<~HBS
   {{#if state.category}}
     {{attach widget="category-display" attrs=(hash category=state.category someNumber=123 someString="wat")}}
   {{/if}}
+  {{#unless state.hello}}
+    XYZ
+  {{/unless}}
 HBS
 
 ctx = MiniRacer::Context.new(timeout: 15000)