Merge layout header data with page_data

This allows users to set default titles in templates.

I'm currently using this to get titles working in pagination.
Unfortunately, pagination pages don't have backing pages so there is no
better place to put the title (except in the config but that's even more
hacky)
Make dates formattable

This commit makes the date format customizable via the `date_format`
option. While this option is site-wide, specific dates can be formated
with `{{#date.strftime}}format_string{{/date.strftime}}`.

closes ruhoh/ruhoh.rb#69
Fix logging error.
Add test cases for date formatting.