Map `C-k` to add link on text fields

[alienbogart]

Is your feature request related to a problem? Please describe.

No.

(optional) Describe the solution you'd like

With text selected, hitting C-k would open a dialog for the user to paste a URL. On pressing Return inside the dialog, the selected text would become a link for the pasted URL.

This is identical to the behavior of the Reddit Enhancement Suite.

(optional) Describe alternatives you've considered

Hitting C-k on selected text could automatically create the link with the URL on the clipboard, without the dialog. Ideally, throw an error if the content of the clipboard is not a valid URL.