Using ~~text~~ syntax will strike out the text between the two tildes.
Only missing portion is the terminal rendering of strike through text.
The ansi escape codes for strike through text are \e[9m and \e[29m but
it appears the terminal does not support these. Please correct me if I
am wrong.
I tested that the render_to_terminal function was being called by giving
it bold ANSI escape codes, and that did work so the function is being
called correctly.
This also improves Commonmark coverage, e.g. it fixes tests
HTML_blocks_ex179_2894..2906 and Lists_ex308_5439..5457.
In other words, we go from 271 out of 652 to 273 out of 652.