Implement new feature: g:numbertoggle_min_cols

When g:numbertoggle_min_cols is set, show line numbers if there are at least g:numbertoggle_min_cols available for displaying content. By default this is set to zero and has no impact on existing users.

Setting g:numbertoggle_min_cols=80 for example causes line numbering to be turned off if the window is not able to display 80 characters of text along side the line numbers. When the window becomes larger, the line numbering will automatically appear.

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>