Clarify the blacklist option

Clarify the blacklist option

- Improve wording in the Add-ons Manager. Explicitly say that the list is space
  separated, since several users had problems with that.
- Add default example blacklist patterns. It's much easier to adapt an example
  than to write something from scratch (or copy-paste). The default patterns are
  for example.com and example.org which are reserved for documentation purposes,
  so it doesn't matter that those get blacklisted by default.

See #704.