Pasting URLs onto existing URLs should not generate Markdown link syntax

[jcarpenter]

As-is, we get the following:

1. Copy URL to clipboard: https://github.com
2. Select URL in [Site](google.com)
3. Paste URL.
4. Expected result: [Site](https://github.com)
5. Actual result: [Site]([google.com](https://github.com/jcarpenter/gambier/issues/new))

I think the problem is that our pasting logic is naive. When we paste, if 1) we have a URL in the clipboard, and 2) we have text selected, it generates a new Markdown link. We need to be more discriminating. If we have a URL selected, for example, we should just paste the plain text.