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
direnv_apply_dump: run the dump command given in <argv>, rather than taking a file containing dump output
direnv_apply_dump: use $direnv
rename direnv_apply_dump -> direnv_load
Merge pull request #90 from gfxmonk/direnv_apply_dump

Add direnv_apply_dump function to stdlib. Thanks @gfxmonk !