Add extension list to mapping

lua/SpecSwitcher/ExtensionMap.lua

@@ -4,6 +4,7 @@ function ExtMap:new (o)
     o = o or {}
     setmetatable(o, self)
     self.__index = self
+    self["list"] = {}
     return o
 end
 
@@ -23,6 +24,21 @@ function ExtMap:add_single (in_ext, out_ext)
             self[in_ext] = {out_ext}
         end
     end
+
+    local in_exists = false
+    for _, exts in pairs(self["list"]) do
+        if exts == in_ext then
+            in_exists = true
+        end
+    end
+
+    if in_exists == false then
+        table.insert(self["list"], in_ext)
+    end
+end
+
+function ExtMap:list_exts()
+    return self["list"]
 end
 
 function ExtMap:add (in_exts, out_exts)