Added `export` alias for set global variable, made for sh/bash/zsh compatibility.

fish-shell already have `setenv` alias for zsh compatibility, hence this is `export` alias for `sh` `bash` `zsh` compatibility.

It's useful for some tools like `ssh-agent` or `boot2docker shellinit` that display `export KEY=VALUE` .

Signed-off-by: Rack Lin <racklin@gmail.com>