Support pasting URLs over selection text in the textarea editor. Does
not work in EasyMDE and I don't intend to support it. Image paste works
as usual in both Textarea and EasyMDE.
The new `replaceTextareaSelection` function changes textarea content via
[`insertText`](https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand#using_inserttext)
command, which preserves history, e.g. `CTRL-Z` works and is also
demostrated below. We should later refactor the image paste code to use
the same function because it currently destroys history.
Overriding the formatting via `Shift` key is supported as well, e.g.
`Ctrl+Shift+V` will insert the URL as-is, like on GitHub.
wxiaoguang