FEATURE: Allow string theme settings to display with multiple lines

To use, add `textarea: true` to the theme settings.yml. For example:

```
my_setting:
  default: "some string"
  textarea: true
```

lib/theme_settings_parser.rb

@@ -38,6 +38,8 @@ class ThemeSettingsParser
       opts[:list_type] = raw_opts[:list_type]
     end
 
+    opts[:textarea] = !!raw_opts[:textarea]
+
     opts
   end