add: the missing vi-jump-previous command and binding.

[sakurawald]

The vi-jump-previous command is a useful command, to jump between current point, and previous point.

A use-case: you are writing a function b, which calls the function a, you need to jump between the definition of a and b. Just double pressed the ' key, it will works.

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The implementaion is easy: We can compose the vi-jump-back command and define-motion.
The effect is to define a new motion-command, its function is vi-jump-back.
But because it is a motion-command, so the vi jumplist facility will save current-point for us automatically.