- Support for leave event, via DOTENV_FILE_LEAVE and _dotenv_event (can
use DOTENV_FILE_LEAVE=$DOTENV_FILE_ENTER).
- Support for searching upwards for $DOTENV_FILE_ENTER (#3).
- Source .env only once per session, but re-source when leave events
are enabled (#1).
- Trigger the machinery when the script gets sourced, for the current
dir (#2).