Release 1.2.0

• Minimum Ruby version now 2.6.
• Use http://unlicense.org/ for doap:license, in the absense of anything more specific.
• Improve documentation on dialects.
• Some improvements for Scheme sharp sequences.
• Re-do support for proper IRI serialization by reading prefixes and base expressions and propgating to lower elements to create PNames when serializing only if appropriate. Consolodates the contents of writer.rb into extensions.rb. (For #20.)
• Add to_sxp extensions from SPARQL.
• Support Common Lisp vectors (#(foo)) using the Ruby Vector class. (For #16.)
• Use generic **options instead of explicit keyword arguments for most to_sxp methods.