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.
Merge pull request #177 from Stebalien/formattable-dates

Make dates formattable