add Fish version of working dir change function (#75)

* add Fish version of working dir change function

* Fix error with the two conditions

Not empty part is important otherwise arguments like -help or -doc, when lf is
not started actually and there is no *last-dir* will result the `dir` variable
empty and cd to home as `cd` changes directory to home.

etc/lf.fish

@@ -0,0 +1,18 @@
+# Change working dir in Fish to last dir in lf on exit (adapted from Bash version)
+#
+# You need to copy the content of this file to ~/.config/fish/config.fish
+#
+
+function lf
+    set tmp (mktemp)
+    command lf -last-dir-path=$tmp $argv
+    if test -f "$tmp"
+        set dir (cat $tmp)
+        if test -n "$dir"
+            if test "$dir" != (pwd)
+                cd $dir
+            end
+        end
+    end
+    rm -f $tmp
+end