The initial release of `defaulteditor`.
Adds the basic functionally:
- `file_must_exist(filepath, create_file=False)`
- `delete_file(filepath, ignore_missing=False)`
- `editor_open(filepath, create_file=False)`
- `hook_change(filepath, create_file=False, poll_delay=0.5)`
- `start`, `join`, `wait`
docstrings and so on are currently missing.