Implement parser for else chaining of helpers

Allows users to chain multiple helpers together using their inverse callbacks. I.e.

```
{{#if foo}}
{{else if bar}}
{{else}}
{{/if}}
```

The control flow here effectively causes the helpers to be nested. The above is actually syntactic sugar for this:

```
{{#if foo}}
{{else}}
  {{#if bar}}
  {{else}}
  {{/if}}
{{/if}}
```

Any helper may be used in this manner, the only requirement is they support normal calls and inverse calls.

Introduces a breaking change in that `{{else foo}}` may no longer be used as a root level operator. Instead `{{^foo}}` must be used.

Fixes #72.

1 files changed, 8 insertions, 2 deletions

diff --git a/spec/parser.js b/spec/parser.js
index 131160a..3686363 100644
--- a/spec/parser.js
+++ b/spec/parser.js
@@ -116,6 +116,10 @@ describe('parser', function() {
     equals(ast_for("{{#foo}} bar {{else}} baz {{/foo}}"), "BLOCK:\n  {{ ID:foo [] }}\n  PROGRAM:\n    CONTENT[ ' bar ' ]\n  {{^}}\n    CONTENT[ ' baz ' ]\n");
   });
 
+  it('parses multiple inverse sections', function() {
+    equals(ast_for("{{#foo}} bar {{else if bar}}{{else}} baz {{/foo}}"), "BLOCK:\n  {{ ID:foo [] }}\n  PROGRAM:\n    CONTENT[ ' bar ' ]\n  {{^}}\n    BLOCK:\n      {{ ID:if [ID:bar] }}\n      PROGRAM:\n      {{^}}\n        CONTENT[ ' baz ' ]\n");
+  });
+
   it('parses empty blocks', function() {
     equals(ast_for("{{#foo}}{{/foo}}"), "BLOCK:\n  {{ ID:foo [] }}\n  PROGRAM:\n");
   });
@@ -147,8 +151,10 @@ describe('parser', function() {
   it('parses a standalone inverse section', function() {
     equals(ast_for("{{^foo}}bar{{/foo}}"), "BLOCK:\n  {{ ID:foo [] }}\n  {{^}}\n    CONTENT[ 'bar' ]\n");
   });
-  it('parses a standalone inverse section', function() {
-    equals(ast_for("{{else foo}}bar{{/foo}}"), "BLOCK:\n  {{ ID:foo [] }}\n  {{^}}\n    CONTENT[ 'bar' ]\n");
+  it('throws on old inverse section', function() {
+    shouldThrow(function() {
+      equals(ast_for("{{else foo}}bar{{/foo}}"), "BLOCK:\n  {{ ID:foo [] }}\n  {{^}}\n    CONTENT[ 'bar' ]\n");
+    }, Error);
   });
 
   it("raises if there's a Parse error", function() {