Custom icon for Safari pinned tabs

Currently, a Safari Pinned tab with aha.io will get a generic 'A' icon (from Aha.io). This is especially confusing because all Atlassian products (Jira, Confluence) get the same icon.

It would be nice if it would get the '!' icon instead, so I can distinguish the tabs better.

Documentation from Apple on how to specify the icon:
https://developer.apple.com/library/ios/documentation/AppleApplications/Reference/SafariWebContent/pinnedTabs/pinnedTabs.html