Add a 'capture' jinja2 filter

This filter returns the string captured by an arbitrary regex capture group.

For example, if 

    {{ foo_string }} 

gives us 

    abc1234def

then

    {{ foo_string | capture('abc(\d+)def') }}

gives us

    1234

and

    {{ foo_string | capture('abc(\d+)(\w+)', 2) }}

gives us

    def