ivy-occur-mode: new commands on "j", "k", "c"

ivy-occur-mode: new commands on "j", "k", "c"

* ivy.el (ivy-calling): Make variable buffer-local.
(ivy-occur-toggle-calling): New command bound to "c".
(ivy-occur-next-line): New command bound to "j".
(ivy-occur-previous-line): New command bound to "k".

This makes `ivy-occur' much more convenient, instead of "gjgjgjg", just
"cjjj". Especially good for commands that change the contents of the
other window, like `describe-function' or `counsel-git-grep'.

Example:

- "C-h f" (`describe-funtion')
- "run" (`self-insert-command')
- "C-c C-o" (`ivy-occur'); "C-o u" also works.
- "cjjjjkkkk"