Add more less-compatible keybindings

[hupfdule]

The following keybindings were added:

 - sync:           [r], [R], [ctrl+r]
 - down:           [e], [ctrl+e], [j], [J], [ctrl+j]
 - up:             [y], [Y], [ctrl+y], [k], [K], [ctrl+k]
 - page down:      [space], [alt+space], [f], [z]
 - page up:        [b], [alt+v]
 - half page down: [u]
 - half page up:   [d]

[z] differs a bit from its less-functionality as it doesn't
support the leading count (as ov doesn't accept such a count).

[noborus]

Thank you for the pull request.
I want to avoid making all default keybindings like less
(It will assign new and frequently used keys to hard-to-press keys).
I'm sorry, but I want to have a config file template for less users.

[noborus]

I don't think it's correct to make the default keybindings like less.
Too many keys are assigned to a function.
I think this is because less was wrong.

I would like to have a config file for less users.