Cache the config file

This commit caches the config file the first time it is read. It also
adds a special case to the resource watcher to reload the config on
change. While a separate config resource could be added, this seems like
overkill.

This halves the compile time on my system and should help fix
ruhoh/ruhoh.rb#166.
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.