Artwork_save file naming.

Added an extra parameter to the "artwork_save" function to be able to give the file a specific name. If no name is given, the file will still be called "artwork.png"

pyatv/scripts/atvremote.py

@@ -406,11 +406,11 @@ async def cli(self):
                 self.args, command, self.atv, self.storage, self.loop
             )
 
-    async def artwork_save(self, width=None, height=None):
+    async def artwork_save(self, width=None, height=None, file_name="artwork"):
         """Download artwork and save it to artwork.png."""
         artwork = await self.atv.metadata.artwork(width=width, height=height)
         if artwork is not None:
-            with open("artwork.png", "wb") as file:
+            with open(f"{file_name}.png", "wb") as file:
                 file.write(artwork.bytes)
         else:
             print("No artwork is currently available.")