Added a number of features to pandoc_reader

[nandac]

The pandoc_reader has been updated to be in line with Pandoc 2.9.2.1. The following features have been added:

• Added ability to interpret YAML metadata placed at the top of the file (this is mandatory)
• Added ability to stop Pelican directives like {static}, {attach} and {filename} from being encoded in output.
• Added error checking for new features above.
• Fixed unbound local error issue if an empty file is passed to the reader.
• Code has been brought up to current python standards.
• Readme has been updated with some extra information