Feat: Implement iterating over explicit list.

When using h2o I found it useful to be able to use the {% for %} block to iterate over a list
given explicitly in the template, e.g.:

    {% for year in 2014, 2015, 2016, 2017 %}
        See <a href={{year}}.html>here</a>
    {% endfor %}

This commit implements this feature by allowing the for to iterate over a comma-separated list of values.