* Docs for permissions: in metadata, refs #1636
* Refactor default_permissions.py to help with implementation of #1636
* register_permissions() plugin hook, closes#1939 - also refs #1938
* Tests for register_permissions() hook, refs #1939
* Documentation for datasette.permissions, refs #1939
* permission_allowed() falls back on Permission.default, refs #1939
* Raise StartupError on duplicate permissions
* Allow dupe permisisons if exact matches