Specify databases to convert; Do not convert all databases by default

My have several non-drupal database connections configured in the settings.php $databases array. These additional connections are used in some custom modules and it works great. The default drush command "utf8mb4-convert-databases" will detect and attempt to convert all of these databases, even though I only want/need the drupal one converted. Converting the non-drupal databases can cause unforeseen problems. In addition, if you have a slave database connection setup, you don't want to convert that because it supposed to be replicated from master (although the slave db connection should be read only to begin with).

Possible solutions:
1. Only convert the ['default']['default'] database.
2. Specify which databases to convert
3. Automatically detect if the target database is a drupal install, and convert only those ones.

In the meantime, I'm just going to hack the code to only convert ['default']['default'].