Add autoenv command to manage autoenv's

usage: autoenv [-h] {init,edit,deinit}

options:
        -h, --help  show this help message and exit

commands:
        init        add .enter and .exit scripts in current directory
        edit        edit .enter and .exit scripts in current directory
        deinit      remove .enter and .exit scripts in current directory

Fixes #9.

autoenv/autoenv.zsh

@@ -42,7 +42,7 @@ commands:
       fi
       # If vim exists, edit enter and exit scripts.
       if which vim &> /dev/null; then
-        vim -p $PWD/.enter $PWD/.exit
+        vim -p .enter .exit
       else
         echo 'vim not found'; return 1
       fi ;;