Hey, I finally got to working on this a bit, and have uploaded a new version.
Key changes include some parsing improvements, quick hyperlink buttons to documentation, and configuration fixes. I also thoroughly tested this time to make sure the vsix package works properly with VS2015 and 2017, so hopefully there should not be any errors involving that anymore.
See first post for instructions if needed. The changelog is below:
- The edit specifier command now starts parsing from the beginning of the line instead of current caret position. This way, the command can be invoked anywhere within a specifier provided no other non-whitespace characters appear before the specifier definition
- Added UINTERFACE
- Added 2 links at the bottom of the editor window that open the doc page for the current specifier, and the other opens documentation for metadata specifiers (no tooltips for now, sorry!)
- Fixed configuration saving and loading. You will now be able to safely edit (add/remove) specifiers and/or rearrange their order at your leisure in the Options dialog of Visual Studio (meta specifiers cannot be modified currently)
- Fixed editor not respecting upper/lowercase letters in edit values (e.g. the value for Category would always be lowercase no matter what you had entered)
- Existing specifiers can now be parsed regardless of case (so ‘category’, ‘CATEGORY’, and ‘Category’ will be parsed the same, although the editor will still do first upper case letter when generating it)
- Fixed build compatibility problem with 1.3 that did not allow the extension to work properly in VS2017 in some cases
Enjoy. 
And as usual, any feedback is welcome.