Support custom URLs for subject image artwork

[aaron-meyers]

It would be nice to allow a user to provide their own subject artwork rather than only sourcing from the community project. I've implemented a variant of the community userscript that does this by searching for image URLs in the notes field. I'm happy to work with the community project to move this capability into the official community userscript before adopting it here in Tsurukame.
 
Part of #689 support for WaniKani Image Mnemonics subject artwork.