Make g:ale_virtualtext_cursor buffer local

It'd be nice to be able to selectively disable virtualtext in some buffers. My use case is that I'm writing prose in markdown, and while writegood's suggestions are handy, they're not something I want to see while writing, and are better viewed in the quickfix list/status menu.

Something like the ability to set `b:ale_virtualtext_cursor` for specific buffers would be nice.