Don't start linting if a file is opened only in 'previewwindow'

It would be nice to add an option to skip linting if a file is opened only for preview. I.e. if one starts `unite.vim` to find something and quickly previews a bunch of files that are closed almost instantly, there's no point to start linting them. So, the algorithm would be something along these lines:
* `if (disable linting in preview) && (file is opened only in one window) && (that window has "previewwindow" option set) then ALEDisableBuffer`
* `if (file is being opened in a window without "previewwindow" option) && (ALE is enabled) then ALEEnableBuffer`

Thanks for the great plugin BTW :)