this fork adds ability to use 'sourcecode' directive with
explicit naming of 2 languages: verilog, systemverilog instead of
v and sv.
Old aliases (v,sv) for languages are kept unmodified.
The main reason to use full language name is because
it is explicit, more readable in ReST sources;
most language has two or more aliases, e.g 'python' and 'py' etc.