feat(bqf.qfwin.handler): allow to use function as opener

[przepompownia]

For example

local function tabDropHandler()
  require('bqf.qfwin.handler').open(true, function(fname)
    customTabDrop(fname)
  end)
end
local function configureWindow()
  vim.keymap.set({'n'}, '<CR>', tabDropHandler, {buffer = true})
end

local augroup = vim.api.nvim_create_augroup ('BqfMappings', {clear = true })
vim.api.nvim_create_autocmd ('FileType', {
  group = augroup,
  pattern = 'qf',
  callback = configureWindow,
})

https://github.com/kevinhwang91/nvim-bqf/issues/45