describe works on installed packages

also modified list_db_packages to return all packages by default, but with a
skip_installed to skip listing installed packages

pkg/omf/cli/omf.list_db_packages.fish

 # List all packages available to install from the registry.
-function omf.list_db_packages
+function omf.list_db_packages -a skip_installed
   for item in (basename $OMF_PATH/db/pkg/*)
-    contains $item (basename {$OMF_PATH,$OMF_CONFIG}/pkg/*); or echo $item
+    if  begin
+          test -z $skip_installed
+          or not contains $item (basename {$OMF_PATH,$OMF_CONFIG}/pkg/*)
+        end
+      echo $item
+    end
   end
 end

pkg/omf/completions/omf.fish

@@ -16,7 +16,7 @@ complete -c omf -n "__omf.opt_is q query"         -a (printf "%s " (set | awk '{
 complete -c omf -n "__omf.opt_is r rm remove"     -a (printf "%s " (omf.list_local_packages) (omf.list_installed_themes))
 complete -c omf -n "__omf.opt_is d desc describe" -a (printf "%s " (omf.list_db_packages))
 complete -c omf -n "__omf.opt_is c cd"            -a (printf "%s " (omf.list_db_packages))
-complete -c omf -n "__omf.opt_is i install"       -a (printf "%s " (omf.list_db_packages))
+complete -c omf -n "__omf.opt_is i install"       -a (printf "%s " (omf.list_db_packages "skip installed packages"))
 complete -c omf -n "__omf.opt_is t theme"         -a (printf "%s " (omf.list_themes))
 complete -c omf -n "__omf.opt_is help"            -a "install theme remove update list describe cd new submit query destroy"
 

pkg/omf/omf.fish

@@ -89,7 +89,7 @@ function omf -d "Oh My Fish"
 
     case "i" "install" "get"
       if test (count $argv) -eq 1
-        omf.list_db_packages | column
+        omf.list_db_packages "skip installed packages" | column
       else
         omf.install_package $argv[2..-1]
         refresh