Support for the boolean() filter

[digitalresistor]

As I am going through and trying to use this, it would be great if there was a standard boolean filter that takes in text and returns true for values like:

• yes
• true
• True

and then false for string values like this:

• false
• False
• no

Although I am not sure if that is what the Jinja2 filter does, or if I am thinking of the ansible bool filter instead...

[mitsuhiko]

I will add this for compatibility but in practical terms it's rarely useful as {% if foo %} is the same as {% if foo|bool %}. I do see it being useful for ansible type uses though and it will become more useful if we do #50.