Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
`"stretch"` is not a valid value for `pack` https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Attribute/pack Did you mean `"center"`? (though I don't think you need `pack` at all)