scripts: Add nsxiv-rifle script

Opens basically any path, local or remote in nsxiv.
Can take multiple paths to open all of them.

Additionally, alias whichever version of the image viewer is available
on the system as `iv`, using `nsxiv-rifle`, `nsxiv`, `sxiv` in order of
preference.

sh/.config/sh/alias

@@ -93,6 +93,17 @@ if exist vifm; then
     alias vmm='vifm ${PWD}'
 fi
 
+# nsxiv image viewer
+if exist nsxiv; then
+    if exist nsxiv-rifle; then
+        alias iv=nsxiv-rifle
+    else
+        alias iv=nsxiv
+    fi
+elif exist sxiv; then
+    alias iv=sxiv
+fi
+
 # python
 if exist ptipython; then
     alias py=ptipython