docs, refactor: update to new aucmd API

• remove vim source code for implementing tidy's autocommands
      and instead use the Lua API from 0.7

    • update README with improved explanation and new installation
      instructions

    Resolves #4

lua/tidy/init.lua

@@ -1,32 +1,37 @@
 local M = {}
 
-function M.tidy_up()
-    -- get tuple of cursor position before making changes
-    local pos = vim.api.nvim_win_get_cursor(0)
+function M.setup()
+    local tidy_grp = vim.api.nvim_create_augroup("tidy", { clear = true })
 
-    -- delete all whitespace, see source 1
-    vim.cmd([[:keepjumps keeppatterns %s/\s\+$//e]])
+    vim.api.nvim_create_autocmd("BufWritePre", {
+        group = tidy_grp,
+        callback = function()
+            local pos = vim.api.nvim_win_get_cursor(0)
 
-    -- delete all lines at end of buffer, see source 2
-    vim.cmd([[:keepjumps keeppatterns silent! 0;/^\%(\n*.\)\@!/,$d]])
+            vim.cmd([[:keepjumps keeppatterns %s/\s\+$//e]])
 
-    -- get row count after line deletion
-    local end_row = vim.api.nvim_buf_line_count(0)
+            -- delete all lines at end of buffer, see source 2
+            vim.cmd([[:keepjumps keeppatterns silent! 0;/^\%(\n*.\)\@!/,$d]])
 
-    --[[
-        if the row value in the original cursor
-        position tuple is greater than the
-        line count after empty line deletion
-        (meaning that the cursor was inside of
-        the group of empty lines at the end of
-        the file when they were deleted), set
-        the cursor row to the last line
-    ]]
-    if pos[1] > end_row then
-        pos[1] = end_row
-    end
+            -- get row count after line deletion
+            local last_row = vim.api.nvim_buf_line_count(0)
 
-    vim.api.nvim_win_set_cursor(0, pos)
+            --[[
+                if the row value in the original cursor
+                position tuple is greater than the
+                line count after empty line deletion
+                (meaning that the cursor was inside of
+                the group of empty lines at the end of
+                the file when they were deleted), set
+                the cursor row to the last line.
+            ]]
+            if pos[1] > last_row then
+                pos[1] = last_row
+            end
+
+            vim.api.nvim_win_set_cursor(0, pos)
+        end,
+    })
 end
 
 return M