Add simple_test for custom test extensions

[mlasevich]

Currently there is a decorator to create a simple filter Jinja extension. This PR adds a similar decorator to create custom test extensions. Included is the decorator, test updatres, and docs updates

[mlasevich]

All the corrections have been done, would it be possible to get this reviewed again :-)

Thank you. :-)

[kurtmckee]

Thanks for addressing my concerns regarding the tests! I've given it a more thorough code review.

@jensens / @ericof, I can't say whether this feature is needed or helpful. Please review this as a new feature and determine whether it should be merged. Thanks!

[kurtmckee]

I think flake8 is upset with this:

Suggested change

	"""
	Register a function for use as a test extension for Jinja templates.
	"""Register a function for use as a test extension for Jinja templates.

[kurtmckee]

I think flake8 is upset with this:

Suggested change

	"""
	Register a function for use as a filter extension for Jinja templates.
	"""Register a function for use as a filter extension for Jinja templates.

[kurtmckee]

Suggested change

	name becomes the filter name
	name becomes the filter name.

[kurtmckee]

This will be rendered in the API documentation. This should be a more complete example, including RST syntax:

Suggested change

	@simple_test
	def mirror(value):
	.. codeblock:: python
	@simple_test
	def mirror(value):
	return value[::-1]

This suggestion applies to the code changes lower in this file, too.

[kurtmckee]

This needs to be a codeblock so that it renders in the API documentation. I don't think pygments has a Jinja lexer specifically, so I'm suggesting text here to avoid incorrect syntax highlighting:

Suggested change

	{{ cookiecutter.param | mirror }}
	.. codeblock:: text
	{{ cookiecutter.param | mirror }}

This suggestion applies to the code changes lower in this file, too.