Implement `pos = 'center'` option

[jesseleite]

• [x] Implement pos = 'center' option.
  • You had this as a TODO.
  • I had to rearrange a few things, as I needed access to width and height before setting line and col.
  • The centering logic is ripped from Telescope; You ever heard of Telescope?
  • Example usage...

    local win_id, win = popup.create(bufnr, {
        minwidth = config.width or 60,
        minheight = config.height or 10,
        pos = 'center',
        -- ...
    })
    

[tjdevries]

If you fix conflicts, I will merge. Sorry for the delay