Security fix

See #23.

TODO:

* Update documentation
* ~~Automatically detect changes to `SUMS=${DIRENV_CONFIG_DIR}/sums/` and update
  environment accordingly~~
* Add tests
* Somehow eliminate obsolete files from `$SUMS` (changing a file requires it to be re-enabled)
* Add the ability to temporarily disable the need to enable a file
* Add the ability to ignore a file (e.g, it's in your path, but you don't control it and you don't want it, and you don't want to be spammed with messages about how to enable it)