Add Copy Image option

[whitecrownclown]

@sindresorhus

This PR follows up on sindresorhus/caprine#943.

A couple of mentions:

• Electron can only copy png or jpg images. I tried with webp, for example, but electron.nativeImage has no support for it. I suppose we could convert to png or jpg.. thoughts?
• I'm not sure if it should support file:// protocol or not
• Used the request module for fetching the image buffer

Thanks!

[sindresorhus]

The implementation here is more complicated than I'd like. I think you instead should open an issue on https://github.com/electron/electron about adding an API to create a native image directly from an URL. They already have all the Chrome machinery to download images, so it would be better if they could handle it. Could be electron.nativeImage.createFromURL(url);. In your use-case and link to this PR. Maybe they can suggest even a better solution. Also mention the WebP problem.

[sindresorhus]

I think you could have spent more time on perfecting this PR. At this point, it would have been much faster for me to just implement it myself.

[whitecrownclown]

Sorry about that, I’ll do better on future PRs.

[sindresorhus]

You still need to do https://github.com/sindresorhus/electron-context-menu/pull/79/files#r312071329