add .luacheckrc and fix linter issues

.luacheckrc

@@ -0,0 +1,13 @@
+-- vim: ft=lua tw=80
+
+-- Don't report unused self arguments of methods.
+self = false
+
+ignore = {
+  "631",  -- max_line_length
+}
+
+-- Global objects defined by the C code
+read_globals = {
+  "vim",
+}