Support importing keys from the clipboard

[orhun]

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

Import operation requires a file that contains the keys. It should also support importing keys using the clipboard.

Describe the solution you'd like

We can add :import-clipboard command or something similar to get the clipboard contents and try to import it.

Describe alternatives you've considered

Add an import mode and specify the way to import things. e.g. pressing i -> f will result in importing a key from a file while i -> p will use the clipboard. (similar to copy mode)