feat(template): add site.robots support

BREAKING CHANGE: Now you can define your custom `robots.txt` records in configuration file:

```yaml
robots:
  - "User-agent: *"
  - "Disallow: /ajax/"
  - "Disallow: /@async"
  - "Disallow: /log/"
```

_app/_posts/note/2014-06-09-creating-themes.md

@@ -203,6 +203,20 @@ Template:
 {{ '{% endif ' }}%}
 ```
 
+### `site.robots`
+
+You can define your own records in `robots.txt`.
+
+Config:
+
+```yaml
+robots:
+  - "User-agent: *"
+  - "Disallow: /ajax/"
+  - "Disallow: /@async"
+  - "Disallow: /log/"
+```
+
 ## Theme Features
 
 There're some features can be integrated into your theme.

_app/robots.txt

@@ -1,5 +1,8 @@
 ---
 ---
 
-User-agent: *
+{% for post in site.robots
+  %}{{ post | strip }}
+{% endfor %}
+
 Sitemap: {{ '/sitemap.xml' | prepend: site.base | prepend: site.url }}

_config.init.yml

@@ -54,6 +54,13 @@ redcarpet:
     - superscript
     - tables
 
+#
+# Jekyll plugins
+#
+
+gems:
+  - jekyll-last-modified-at
+
 #
 # Almace Scaffolding settings
 #
@@ -117,3 +124,13 @@ rss_excerpt:    false
 # Show generator credits
 # Default:      true
 credits:        true
+
+# Custom records in robots.txt
+# For example:
+# robots:
+#   - "User-agent: *"
+#   - "Disallow: /ajax/"
+#   - "Disallow: /@async"
+#   - "Disallow: /log/"
+# Default:      "User-agent: *"
+robots:         "User-agent: *"

_config.yml

@@ -124,3 +124,13 @@ rss_excerpt:    false
 # Show generator credits
 # Default:      true
 credits:        true
+
+# Items in robots.txt
+# For example:
+# robots:
+#   - "User-agent: *"
+#   - "Disallow: /ajax/"
+#   - "Disallow: /@async"
+#   - "Disallow: /log/"
+# Default:      "User-agent: *"
+robots:         "User-agent: *"