template.py: define a `_Template` class instead of two functions

template.py: define a `_Template` class instead of two functions

This patch changes `template.py` so as to offer the `_Template` class
instead of the _create_template() and _render_template() functions.

This makes it possible to move the `is_file_template` parameter from
_render_template() to the template's constructor, as this really is a
template-specific option, not a rendering-specific option.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>