Separate Smartypants somewhat from the HTML renderer. Move its flags
from HtmlFlags to Extensions (probably should be moved to its own set of
flags, but not now). With that done, do a separate walk of the tree and
either run Smartypants processor if it's enabled, or simply escape text
nodes.
The flag `HTML_SMARTYPANTS_ANGLED_QUOTES` combined with `HTML_USE_SMARTYPANTS` configures rendering of double quotes as angled left and right quotes (« »).
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.
