Add nlinum.el
* nlinum.el: Speed up by caching last line-number.
(nlinum--line-number-cache): New var.
(nlinum--after-change, nlinum--line-number-at-pos): New functions.
(nlinum-mode, nlinum--region): Use them.
* nlinum.el: Add global-nlinum-mode and nlinum-format-function.
* nlinum.el (nlinum-format): New custom variable.
(nlinum--region): Change calling convention of nlinum-format-function.
(nlinum-format-function): Change default value accordingly; Use nlinum-format;
Try to generate less garbage.
* packages/nlinum/nlinum.el (nlinum-mode): Don't leave overlays around when
switching major mode.
* packages/nlinum/nlinum.el (nlinum--face-height): New function.
(nlinum--setup-window): Use it.
Fixes: debbugs:17906

* packages/nlinum/nlinum.el (nlinum--setup-window): Don't burp in
non-graphic terminals.