GFM meta parser ignores blank lines; this is non-standard

[sebbASF]

The GFM reader skips blank lines when parsing meta-data.
This is in contrast with the standard Pelican parsers which treat a blank line as the end of the meta-data block.

This matters because:

• it is non-standard
• the parser can treat the start of body content as a meta-data item, if the body starts with something that looks like a keyword. This is unlikely, but not impossible.

It looks as though this behaviour has been relied on in some ASF website sources, in particular the main website.

Some options are:

• just document the behaviour
• add an option to warn if such blank lines are found
• add an option to disable the behaviour

[sebbASF]

Just noticed that some ASF pages contain a line starting with 'Note: '.
There don't seem to be any pages where this is the first non-blank line, but it could easily be the case.

Obviously there are work-rounds, but it might not be obvious to the editor why their page was misbehaving.