Accept pandoc-style YAML frontmatter

Pandoc (and some other tools) have decided to end their YAML frontmatter
with `...` instead of `---`.  In the name of flexibility, this patch
allows either to mark the end of YAML frontmatter.

Example:

```
---
title: No place like home
...
```

middleman-core/lib/middleman-core/core_extensions/front_matter.rb

@@ -119,7 +119,7 @@ module Middleman::CoreExtensions
     # @param [String] content
     # @return [Array<Hash, String>]
     def parse_yaml_front_matter(content, full_path)
-      yaml_regex = /\A(---\s*\n.*?\n?)^(---\s*$\n?)/m
+      yaml_regex = /\A(---\s*\n.*?\n?)^((---|\.\.\.)\s*$\n?)/m
       if content =~ yaml_regex
         content = content.sub(yaml_regex, '')