I18nice

- Rewrote `PythonLoader`
- **(B)** Removed old `error_on_missing_*` settings
- Improved memoization
- Improved file loading and fixed bugs
- Improved exceptions
- Added `reload_everything()`
- **(B)** Removed deprecated `other` plural

- Trying to set inexistent setting will now raise KeyError
- Added custom functions
- Fixed settings not updating properly
- **(B)** Dropped Python 2
- Added `on_missing_*` hooks