Disable when `filetype=` (binary files); use `modifiable`

[hinell]

If binary file is opened, column line is still shown. Would be nice to disable it.

I also would like to suggest to use modifiable win option, to recognize when to enable / disable virt-column. This would be useful when editing man or help files manually so we have columns shown.

Consider this as a feature request. Thanks.

Related: https://github.com/xiyaowong/virtcolumn.nvim/issues/