Implement support for loading and unloading .env files

[simotek]

This provides two new functions for loading and unloading
.env files, tacklebox now also loads any .env file it finds
in the base tackle directory on startup.

This is my first attempt at loading/unloading env files, unfortunatly I had to add a special case for $PATH as fish treats it specially. I am also relying on the new string built in thats part of the fish 2.3 beta as it makes life a lot simpler.

Currently its just loading all the .env files in the tackle directory I have a example here https://github.com/simotek/tackle/tree/simotek-theme eventually ill clean up that theme enough for general use.

I was also thinking of setting it up to load .env files in the theme directory as files like https://github.com/simotek/tackle/blob/simotek-theme/simotekfish-colors.env are probably better as part of the theme.