initial support for fish
Try to add a `direnv hook fish` command
Fixes `direnv hook fish`
Ignore environment variables that start with __fish

I'm seeing environment variables like __fish_bin_dir __fish_datadir
__fish_help_dir __fish_sysconfdir and I don't think it's desirable to change
them.
Threat fish's PATH export specially
merging changes from zimbatm
split the direnv call into a shared function