Allow using a function to exclude filetypes

[ptn]

I use this because it has allowed me to change my config from this:

      exclude_filetypes = {
        "netrw",
        "toggleterm",
        "NeogitCommitMessage",
        "NeogitCommitView",
        "NeogitDiffView",
        "NeogitHelpPopup",
        "NeogitLogPopup",
        -- ...bunch more Neogit buffers
      },

to this:

      exclude_filetype_fn = function(filetype)
        return string.find(filetype, "Neogit")
      end,