Remappable key bindings

[orhun]

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

Making the key bindings configurable is always a plus for the user experience.

Describe the solution you'd like

First, create a struct for the key bindings and update the rest of the application to use this struct while handling the key presses.
Next, add a section to the configuration file (#5) for custom key bindings, and parse them into the struct that is mentioned earlier.

[orhun]

Implemented in 0.9.0: https://github.com/orhun/gpg-tui#customization