fix: Restore support for dictionary-typed theme setup parameter (#162)

* support passing in dictionary as theme definition

Allows setting up lualine with a theme dictionary like:
```lua
setup{
  theme = { normal = {...}; insert = { ... }; }
}
```
this was the original behaviour before 37a3b8cc.

* comment

lua/lualine/init.lua

@@ -279,6 +279,9 @@ local function setup_theme()
       }, true, {})
       theme = require 'lualine.themes.gruvbox'
     end
+  else
+    -- use the provided theme as-is, assuming it's a dictionary
+    theme = config.options.theme
   end
   highlight.create_highlight_groups(theme)
   vim.api.nvim_exec([[