For loop improvements

[ilyapuchka]

This PR contains several improvements for for tag:

• fixed using spaces between loop variables. Before it was failing parsing.

{% for key, value in dict %}

• added subscripting arrays and dictionaries using loop variable for index or key:

{% for index, value in array %}
{{ index }}: {{ array.index }}
{% endfor %}

{% for key in dict %}
{{ key }}: {{ dict.key }}
{% endfor %}

• added support for iterating arrays with index and value. This (almost) removes need for forloop.counter and forloop.counter0 variables.

{% for index, value in array %}
{{ index }}: {{ value }}
{% endfor %}

• added support for iterating tuples, similar to iterating dictionaries:

{% for label, value in tuple %}
{{ label }}: {{ value }}
{% endfor %}

{% for label in tuple %}
{{ label }}: {{ tuple.label }}
{% endfor %}

• add support for subscripting tuples with labels and indexes. I.e. tuple (one: 1, two: 2) can be subscripted with labels: {{ tuple.one }} {{ tuple.two }} or with indexes: {{ tuple.0 }} {{ tuple.1 }}

• added break and continue tags for breaking/continuing current loop (I started to implement labeled loops, but it will require a bit more effort).

{% for item in items %}
  {{ item }}
  {% if condition %}{% break %}{% endif %}
{% endfor %}

[ilyapuchka]

@kylef I'll break this into smaller separate prs so that it's easier for you to review, I expected it to be smaller, but it grew fast =))