Add configurable support for angled quotes

The flag `HTML_SMARTYPANTS_ANGLED_QUOTES` was added to Blackfriday on Black
Friday. This configures rendering of double quotes as angled left and right
quotes (&laquo; &raquo;).

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 tongue); he would then configure Blackfriday 
to use angled quotes for the Norwegian section, but keep them as reqular 
double quotes for the English.

This commit adds configuration support for this new flag, configuration that
can be set in the site configuration, but overridden in page front matter.

Fixes #605
I’m not fond of `Markdown` being the head item for this, to be honest. The truth is that `FootnoteAnchorPrefix` and `FootnoteReturnLinkContents` are both `Markdown` flags, to be honest. It doesn’t feel that `Markdown` would by necessity be `map[string]bool` otherwise.

This may be a larger complaint about the fact that all of the options are essentially global, and that it might be nice if `viper` had support nested keys (e.g., `viper.SetDefault("Markdown.angledQuotes", false)`). It isn’t something that I *have* to have, but there are starting to be enough toggles in Hugo that a reorganization before 1.0 is released is probably a good idea, even if it causes pain in migration.