refactor!: disable netrw by default (#155)

If you use oil and you want to still use netrw, set
`default_file_explorer = false`.

It is nonsensical to both use netrw _and_ have oil hijack directory
buffers (which was the case for the default config). It also causes
undefined behavior and bugs. When `default_file_explorer = true` (the
default) oil will now disable netrw for you.

doc/oil.txt

@@ -14,6 +14,9 @@ OPTIONS                                                              *oil-option
 
 >
     require("oil").setup({
+      -- Oil will take over directory buffers (e.g. `vim .` or `:e src/`)
+      -- Set to false if you still want to use netrw.
+      default_file_explorer = true,
       -- Id is automatically added at the beginning, and name at the end
       -- See :help oil-columns
       columns = {
@@ -38,8 +41,6 @@ OPTIONS                                                              *oil-option
         conceallevel = 3,
         concealcursor = "n",
       },
-      -- Oil will take over directory buffers (e.g. `vim .` or `:e src/`
-      default_file_explorer = true,
       -- Restore window options to previous values when leaving an oil buffer
       restore_win_options = true,
       -- Skip the confirmation popup for simple operations