add "apply_data" option.

Hiki markup (*) uses "{{...}}" syntax for plugin and Jekyll (Liquid) also
uses "{{...}}" syntax for variable expansion. When I use Hiki -> HTML
converter, the "{{...}}" syntax is conflicted. I can't resolve the conflict
in source file because Liquid doesn't have escape syntax. So this change
adds "apply_data" page data option that controls whtether Liquid is used or not.
When I add the following header, I can solves the conflict:
<pre>
---
apply_data: false
---
</pre>

(*) http://hikiwiki.org/en/TextFormattingRules.html#l19