Add option to attempt splitting metadata using `\n\n` if metadata delimiters are missing

[Holzhaus]

This might work for very simple metadata, like:

title: Foo bar
date: 2020-01-01 10:00:00

This is text

If the metadata delimiters are not found and the option is enabled, the plugin could do something like this:

meta, sep, content = text.partition("\n\n")
try:
    metadata = yaml.load(meta, Loader=...)
except yaml.error.YAMLError:
    content = text
    metadata = {}
else:
    # Prevent false-positives if the text does not begin with a metadata block
    if isinstance(metadata, str):
        metadata = {}
        content = text

self.md.Meta = metadata
return content

[sivakov512]

I prefer a strict approach, such as with a metadata\yaml separator. This is more obvious and avoids unexpected behavior.

[Holzhaus]

I agree that this should generally be preferred. However, sometimes existing data is in the old format. Have you looked at the linked PR #21? It makes this behavior configurable (off by default).

Use case: I migrated @mixxxdj/website to Pelican from cactus and used your module to add support for extra metadata. But most of the old blog posts lack the YAML delimiters and changing them all would be really annoying.

[sivakov512]

Okay, use case sounds good.