TeslaUnread provides unread count badges for Nova Launcher Prime and WidgetLocker. It will not work with other desktops or lockscreens.

Unread counts are natively supported for the most popular chat and email apps, such as Hangouts, Whatsapp and all the apps shown in the screenshot.
Additionally on Android 4.3+ TeslaUnread can watch the notification bar to extract unread counts from virtually any app.

Enable TeslaUnread in Nova Launcher or WidgetLocker Settings > Unread Counts, here you may also customize the style of the badge, such as the size and colors.
From version TeslaUnread 5.0.8:
Simplified user interface
Improved handling of unread counts from notifications
Improved or added support for several more apps
Added crash and usage analytics (and internet permission) to help identify issues and further improve TeslaUnread
5.0.6 March 18
Fixes for K9/Kaiten on Marshmallow
All versions
March 6, 2017
February 23, 2017
February 20, 2017
February 10, 2017
March 18, 2016
March 16, 2016
March 14, 2016
March 12, 2016
March 7, 2016
February 26, 2016