Add strikeout (del) tag

+ trivial emphasis bugfix
Added typographer draft
Started adding remarkable-specific tests
Replaced `assign` helper
Typographer: enchanced plain rules
Added links autoconversion
Code structure and options refactoring
Demo update
Docs update
Splitted typographer tests
Optimize backtracking in links
Add more tests

+ a benchmark case
Forbid nested reference links
Docs minor update
demo update
Add remarked tests for tables and fix renderer accordingly