Add a localize template tag

[ellmetha]

Description

Let's introduce a new localize template tag allowing to perform basic localization operations in templates.

Context

Marten relies on crystal-i18n in order to power its internationalization features. The template engine presently provides a translate tag to perform translations within templates. That said, there is no easy way to perform localizations in templates at the moment (that is, without having to implement a custom template tag).

Proposition

Let's introduce a new localize tag template that will allow to perform the localizations that are already provided by crystal-i18n (that is, localizing dates, datetimes, and numbers).

Concretely, it should be possible to localize such values using the default format:

{% localize 100000 %}
{% localize created_at %}

We should also make it possible for users to explicitly define the localization format that they want to use with a dedicated format argument:

{% localize 100000 format: "custom" %}
{% localize created_at format: "short" %}