Fix npm link, add wikipedia Talk support.

Delegate all properties to base class for initialization.
This is done so that derived classes can remain purely declarative.

TODO: Eliminate setTimeout workaround for wikipedia.fly.
Could not get onload or readystatechange to work righ now.
TODO: Would be nice if base classes could also be declarative, at
least partially to make contributions easier.
Add new menu entry "Extend Issue Pigeon" for user to add definitions.
This adds preference KNOWN_SITES_EXTENSIONS.

Add constructors lookup object so knownOrigins can use constructor
names and be stringified when needed.
Update knownOrigins accordingly.
Add test for new content script data/extendKnownSites.js.