option to enable spell checking (#815)

lua/default-config.lua

@@ -13,6 +13,8 @@ O = {
   clipboard = "unnamedplus",
   hidden_files = true,
   wrap_lines = false,
+  spell = false,
+  spelllang = "en",
   number = true,
   relative_number = false,
   number_width = 4,

lua/settings.lua

@@ -56,3 +56,5 @@ opt.relativenumber = O.relative_number -- set relative numbered lines
 opt.numberwidth = O.number_width -- set number column width to 2 {default 4}
 opt.signcolumn = "yes" -- always show the sign column, otherwise it would shift the text each time
 opt.wrap = O.wrap_lines -- display lines as one long line
+opt.spell = O.spell
+opt.spelllang = O.spelllang