SmartyPants quote handling isn’t actually compliant with original implementation

[codyrobbins]

Here are a few counterexamples which John Gruber’s original Perl implementation of SmartyPants correctly handle:

• ma'am
• o'clock
• s'étendre
• à l'extrême
• j'apprecie tes efforts
• d'une manière soignée
• c'est pour cela que

Looking at ext/redcarpet/html_smartypants.c, this appears to be due to the hard-coded recognition of a small subset of English contractions in smartypants_cb__squote. Is there a way to handle these single quotes more generally while maintaining the same performance?

[vmg]

Ironically, I tend to forget there are languages other than English. Looking into a fix.

[robin850]

Duplicate of #57 and #66.