Implement jinja templates

[elchininet]

This pull request allows one to use also Jinja templates. The next blocks will render the same:

JavaScript template (the template is rendered locally)

[[[ "My Home " + new Date(states("sensor.date_time_iso")).toLocaleTimeString().slice(0, 5) ]]]

Jinja template (the template is rendered in the back-end)

My Home {{ as_timestamp(states('sensor.date_time_iso')) | timestamp_custom('%H:%M') }}

[coveralls]

coverage: 100.0%. remained the same
when pulling 6cf1b5c on implement_jinja_templates
into d46bb1c on master.