Add support for angled, double quotes

The flag `HTML_SMARTYPANTS_ANGLED_QUOTES` combined with `HTML_USE_SMARTYPANTS`
configures rendering of double quotes as left and right quotes (&laquo;
&raquo;).

The SmartyPants documentation mentions a special syntax for these, `<<>>`, a
syntax neither pretty nor user friendly.

Typical use cases would be either or, or combined, but never in the same
document. As an example would be a person from Norway; he has a blog in both
English and Norwegian (his native tounge); he would then configure Blackfriday
to use angled quotes for the Norwegian section, but keep them as reqular
double quotes for the English.

If the flag `HTML_SMARTYPANTS_ANGLED_QUOTES` is not provided, everything works
as before this commit.