Add direnv_apply_dump function to stdlib

It works with the new `direnv apply-dump` command to
convert a dumped environment (typically from a different process)
into a series of `export` statements which are then evaluated
in the current process.

Fixes #85, obsoletes #89.

Unfortunately the simple `direnv_dump | direnv_apply_dump` idea doesn't work because of subshells, so you need to use pipe redirecion (or whatever the `<(command)` syntax is called). I didn't make `direnv apply-env` accept a shell name, because as far as I can tell it's only ever used from bash - but this could be added in the future.