scripts: Change nxsiv-rifle to uoeia

I rewrote and extended the nsxiv-rifle script recently, and this change
now: removes the old script, adds the new one as a submodule into the
repository and links a working binary into the path.

The new script is essentially still just an image grabber-and-opener,
but supports more image viewers, is a little more advanced and can for
example replace arbitrary parts of urls (and is documented a little more
nicely).

Make qutebrowser use new script and remove leftover references to
nsxiv-rifle. Qutebrowser also gets a third option of viewing images, now
there are:

,i -> open selected link in imageviewer
,I -> open current page in imageviewer
,<C-i> -> open selected image in imageviewer

sh/.config/sh/alias

@@ -105,16 +105,10 @@ if exist imv-folder; then
             imv "$@"
         fi
     }
-elif exist imv; then
-    alias iv=imv
-elif exist nsxiv; then
-    if exist nsxiv-rifle; then
-        alias iv=nsxiv-rifle
-    else
-        alias iv=nsxiv
-    fi
-elif exist sxiv; then
-    alias iv=sxiv
+else
+    for cmd in imv nsxiv vimiv sxiv feh; do
+        if exist "$cmd"; then alias iv="$cmd"; break; fi
+    done
 fi
 
 # python