Get maktaba version from addon-info.json and add Plugin.AddonInfo()

This moves the authoritative maktaba version number into addon-info.json and changes the code for `maktaba#VERSION` and `maktaba#IsAtLeastVersion()` to fetch it from there at runtime.

Also involved adding a `Plugin.AddonInfo()` that allows access to the contents of addon-info.json for a plugin.

See #32 for context.