Add 'quantum' filter, providing mechanism to format quantums consistently.

Used like this:

    {{ my_date | quantum('date') }}

The first argument is the name of a format, which is defined in configuration.

Trex has three defaults that should be ok for most sites:

[quantum]
format_date = %-e %b %Y
format_time = %H:%M
format_datetime = %-e %b %Y %H:%M

These can be overridden in default.ini, and new ones added.