Add `jinja_render' command (and corresponding runner class) to be able
to produce output files using jinja2 template engine from a `run' section of
assistant YAML file.

Usage example maybe found here:
https://github.com/zaufi/devassistant-data/blob/master/assistants/crt/python/kate-plugin.yaml
Fix error in Python 3.3:
    TypeError: 'dict_keys' object does not support indexing
Check if output parameter of a template key has any placeholders and check args[output] key only in that case.
redesign jinja_renderer according notes 1 and 2 of the PR #136
split Jinja2Runner.run() into few smaller methods (last fix for PR #136)
Merge pull request #136 from zaufi/master

New command runner: Jinja2 renderer