Feat: Implement iterating over explicit list.

[jonathanverner]

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.

[speedmax]

I am not sure about comma list. How does similar templates handle this? (django, jinja, liquid)

[jonathanverner]

It seems that django and liquid do not have this feature, jinja has the list in square brackets, values
separated by commas, e.g.:

{% for  month in ['Jan','Feb','Mar'] %}
    {{ month }}
{% endfor %}

A little bit more to type, but very reasonable. Also, if string literals were to be supported, the implementation would have to make sure to ignore commas in the literals (a bit more work
than just calling the split function)