Add Phlex components to Tailwind configuration

Phlex components are Ruby files, so they have a `.rb` extension.

That's why, to be able to use Phlex components with Tailwind, we need
to add the `.rb` extension to the list of files that Tailwind should
look at when generating CSS.

This is done in the `config/tailwind.config.js`:

```js
module.exports = {
  content: [
    // ...
    './app/views/**/*.{erb,haml,html,slim,rb}'
  ]
}
```

Thanks to this commit, we automatically add the `.rb` extension to the
list of extensions when it is not already present.

lib/install/phlex.rb

@@ -27,4 +27,12 @@ class ApplicationView < Phlex::View
 	RUBY
 end
 
+tailwind_config_path = Rails.root.join("config/tailwind.config.js")
+
+if tailwind_config_path.exist?
+	insert_into_file tailwind_config_path, after: "content: [" do
+		"\n    './app/views/**/*.rb',"
+	end
+end
+
 say "Phlex successfully installed!"