Fix #317 - Add support jinja2 support to alert_subject

[mrfroggg]

Opened to modification:
I re-used alert_text_type = alert_text_jinja to render alert_subject: reason is, I would not want to add another parameter that most users wouldn't know about.
And, I think that if you are using jinja2 templates for your alert_text, there's a lot of chance you would want to have the same configuration with alert_subject.

And it won't break configs, it will still check for alert_subject_args first.