New extension: strike through with two tildes [jgm/peg-markdown]

See https://github.com/jgm/peg-markdown/commit/7cc5b73
disable strike-through extension by default [jgm/peg-markdown]

See https://github.com/jgm/peg-markdown/commit/9556c54
Add strike-through support for groff output [jgm/peg-markdown]

See https://github.com/jgm/peg-markdown/commit/7748eca
Rewrote Strike parser to avoid exponential blowup. [jgm/peg-markdown]

See https://github.com/jgm/peg-markdown/commit/a035b71
Sp? is redundant… [jgm/peg-markdown]

See https://github.com/jgm/peg-markdown/commit/b220285
update parser interface
parser.leg.go: update
Parser: add MarkdownString