Reconstructed the select field type.
- Tweaked the getHTMLTagAttributesFromArray() method to check if the parsing property can be resolved as a string value.
- Moved the getHTMLTagAttributesFromArray() method to the utility class.
- Reconstructed the select field type.
- Added the ability to create grouped options.
- Added an example of grouped options in the select field in the demo plugin.
Resolves #48