preservationmeter: Using Jinja2 comments.

* Changed HTML comments to Jinja2 comments.

Signed-off-by: Joao Goncalves <jsvgoncalves@gmail.com>
preservationmeter: Added more file extensions.

* Still needs review, but supports much more extensions.

Signed-off-by: Joao Goncalves <jsvgoncalves@gmail.com>