Honor user setting of numberwidth

Only force numberwidth=4 if &numberwidth does not exist at all or is
set to a value smaller than 4
Only show numbers for listed buffers

If we don't check if a buffer is listed, vim-numbertoggle will enable
numbers in buffers where it's not desired to have numbers at all (e.g.
in NERDTree, tagbar, help, etc.)
Revert "Only show numbers for listed buffers"

This reverts commit 52654939e89532c2a125523274ea2958fbfaf983.
Merge pull request #17 from frace/master

Honor user setting of numberwidth