This makes the name of the auth file location more fitting, and uses the
[XDG spec](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables)
for the default location (~/.local/share/autoenv_auth).
When the old setting is used, or the previous default exists, a warning
is given with instructions.
This changes the defaults:
- AUTOENV_ENV_FILENAME: .env_auth => .autoenv_auth
- AUTOENV_FILE_ENTER: .env => .autoenv.zsh
- AUTOENV_FILE_LEAVE: .env_leave => .autoenv_leave.zsh
`.env` is usually used only for key-value pairs for environment
settings, e.g. with foreman. We do not want to interfere with this.
Fixes https://github.com/Tarrasch/zsh-autoenv/issues/31.
