Develop a Microsoft Office app that can parse a YAML file and present the data tags in the 'Insert Merge Field' drop down list of the Mailings (mail merge) tab of Microsoft Word [see attached document for a screenshot].

Alternatively, present the data tags for inserting in a custom toolbar in the Add-ins tab of the Microsoft Office Ribbon.

The YAML data can be in either plain text .txt or Word .docx file format.

The objective is to allow end users to edit their data in YAML files (instead of Word or Excel tables required for mail merge). This then becomes a simple content management system that can harness the familiar interface and formating features of Microsoft Word.