add to DOWNGRADE_LITERAL and make them the same between tsv2owl and tsv2soml

[VladimirAlexiev]

Datatypes rdf:HTML and sysont:Markdown are used to mark text literals that include HTML and Markdown respectively.
(where @prefix sysont: <http://ns.ontowiki.net/SysOnt/>).

• Add these datatypes to the -l "downgrade literals" option
• Ensure the option works the same in tsv2owl and tsv2soml

[VladimirAlexiev]

@SCVFlare This is in the following source locations:

Newer tool, more elaborate:

• https://github.com/VladimirAlexiev/soml/blob/master/tsv2owl/tsv2owl.pl#L8C1-L18C6
• https://github.com/VladimirAlexiev/soml/blob/master/README.org#34-tsv2owl : -l: downgrade literal datatypes

Older tool, less elaborate:

• https://github.com/VladimirAlexiev/soml/blob/master/tsv2soml/tsv2soml.pl#L11
• https://github.com/VladimirAlexiev/soml/blob/master/README.org#32-tsv2soml: -l: downgrade Literals

Would you like to submit a PR to make them the same, and add rdf:HTML and sysont:Markdown ?