GFM parser does not allow numbers in attributes

[sebbASF]

Standard Pelican allows metadata attributes to contain numbers

[sebbASF]

https://python-markdown.github.io/extensions/meta_data/
says:

“The keywords are case-insensitive and may consist of letters, numbers, underscores and dashes and must end with a colon.
The values consist of anything following the colon on the line and may even be blank.

If a line is indented by 4 or more spaces, that line is assumed to be an additional line of the value for the previous keyword.
A keyword may have as many lines as desired.

The first blank line ends all meta-data for the document.”

The parser also allows YAML-style delimiters.

[sebbASF]

Note: Bug #73 means that allowing different characters in keywords may increase the risk of accidentally matching the start of body-content.