add schema-org annotations to html item encoding

[pvgenuchten]

resolves #718

Overview

when encoding is text/html
records.py adds a second record representation in schema.org to record_dict, which is picked up by item template
added 2 mapping functions type and role, not sure if it is the proper place for mapping files?

Related Issue / Discussion

Resolves #718

Additional Information

output can be validated in https://validator.schema.org/

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines