This commits adds flag `HTML_SMARTYPANTS_QUOTES_NBSP` which, when combined with `HTML_USE_SMARTYPANTS` will insert non-breaking spaces between the double quotes and the contained text.
This is mostly relevant for use in French with `HTML_SMARTYPANTS_ANGLED_QUOTES`.
It should not hurt existing code path in the performance department:
```
name old time/op new time/op delta
SmartDoubleQuotes-4 2.58µs ± 1% 2.58µs ± 1% ~ (p=1.000 n=5+5)
name old alloc/op new alloc/op delta
SmartDoubleQuotes-4 5.27kB ± 0% 5.27kB ± 0% ~ (all samples are equal)
name old allocs/op new allocs/op delta
SmartDoubleQuotes-4 13.0 ± 0% 13.0 ± 0% ~ (all samples are equal)
```
Fixes#378
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.
