Replace POST_FORMAT with `handler.format()`

[NGenetzky]

Currently, the package's init enforces the post template, but it would be more flexible if the Handler class could choose how to format the string.

POST_TEMPLATE = """\
{start_delimiter}
{metadata}
{end_delimiter}

{content}
"""
def dumps(post, handler=None, **kwargs):
    ...
    return POST_TEMPLATE.format(
        metadata=metadata, content=post.content,
        start_delimiter=start_delimiter,
        end_delimiter=end_delimiter).strip()