commonmark.js 0.22.0

• Added iframe to whitelist of HTML block tags (as per spec).
• Removed pre from rule 6 of html blocks (see commonmark/commonmark-spec#355).
• Fixed logic error in calculation of offset. Here's a test case this
  fixes:
  `
  % echo -en "1. \t\tthere" | bin/commonmark

  1. ere
     

  `
• Disallow list item starting with multiple blank lines
  (commonmark/commonmark-spec#332).
• Fixed bug in list-start parser (#60).
• Changed smart quote algorithm to agree with cmark (#61). The change only
  involves double quotes, which now need to be both left flanking and
  not right flanking to count as "can open."
• Fix replacement of multiple apostrophes in the same paragraph with
  --smart (Brandon Frohs).
• make bench: renice to -10 rather than 99.