feat: case insensitive sorting (#429)

* check for sorting option in netrw

* documentation

* refactor: remove sort_ prefix

---------

Co-authored-by: Steven Arcangeli <stevearc@stevearc.com>

README.md

@@ -216,6 +216,8 @@ require("oil").setup({
     -- Sort file names in a more intuitive order for humans. Is less performant,
     -- so you may want to set to false if you work with large directories.
     natural_order = true,
+    -- Sort file and directory names case insensitive
+    case_insensitive = false,
     sort = {
       -- sort order can be "asc" or "desc"
       -- see :help oil-columns to see which columns are sortable