Fix some issues with pasting image urls

[bobjflong]

Thanks for a great package!

I think there might be an issue with pasting image urls. For example, when I copy and paste a URL from the Chrome address bar into the demo, it doesn't seem to convert it to an image:

I think this might be different depending on browser/OS/etc, but for me the transfer type is HTML, not text, so I think that's the problem here. I've made a change here that doesn't use the text transfer type when deciding if the content is a URL to convert.

Additionally I think this caller of asyncApplyChange might be incorrect: https://github.com/ianstormtaylor/slate-plugins/compare/master...bobjflong:BL/fix-paste-images?expand=1#diff-d52935d8da17e52dfbb28f4ad6930467L186

Additionally, I feel that this conversion might be very opinionated. I think it's a normal expectation that a pasted URL wouldn't automatically become a visual image. For that reason, I've added a convertUrlsToImages config item which defaults to true.