Include and require volt function

The volt function "include" always throws exception if template was not found. This pull requests changes this behaviour:

1) new function "require" is added that works as "include" used to work before
2) "include" now ignores missing templates

To achieve this, partial method had to be changed too: an 3rd optional parameter "ignore_missing" was added. This parameter had to be taken into account in simple view, but there it has no effect (partial always throws exceptions on missing files, because it is not supported there)

Usage:
        {% include "partials/nonexistent" %} (never fails)
        {% require "partials/help.volt" %}      (fails if help.volt is missing)
        {{ partial("partials/help", ['a': 'abc'], true) }} (never fails)