A Emacs function for the linum-format variable to highlight the
current line number with a custom face.
Copy this file in a directory which is in the Emacs load-path. Then,
execute the following code either directly or in your .emacs file:
(require 'linum-highlight-current-line-number)
(setq linum-format 'linum-highlight-current-line-number)M-x customize-group and then linum. Finally, Linum Current Line Number Face.