Need a verbose message on duplicated and dropped observations after countryid standardization

[anhqle]

When using psData, the user would frequently see messages such as:

37 duplicated values were created when standardising the country ID with iso2c.
59 observations dropped based on missing values of the standardised ID variable.

I think it would be a good idea to add a verbose option that allows user to know exactly which id has been dropped / duplicated due to transforming which type of country ID to another. It would add a lot of peace of mind. In my experience, I always go back to check what has been dropped manually.

If @christophergandrud think this is a good idea, I would be happy to contribute a pull request.

[christophergandrud]

Sounds good to me.