Adds configurable jinja2 extension loading

Jinja extensions adds features to the jinja2 templating engine. This
patch allows module loading for the templating engine via an
ansible.cfg configuration key (jinja_extensions).
The default behaviour doesn't change (no module loading).
Requested modules can be added coma separated in ansible.cfg

Adds whitespace handling in jinja_extension config

Added whitespace handling in jinja_extension configuration directive, so
things stay safe if user adds spaces around comas in the directives
list.

Adds config example for jinja_extensions

Added config example with multiple extentions for jinja_extensions