Add g:numbertoggle_min_cols setting

When g:numbertoggle_min_cols is set, the plugin will
only show line numbers if there are at least g:numbertoggle_min_cols.

This allows one to have the line numbering turned off when working in
narrow terminals and have it automatically turn on for wider terminals.

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>