Configurable suffix for percentage label in ProgressBar

[priv-kweihmann]

I was playing around with the ProgressBar and used it in tests to display the temperature of some sensors, which is looking great, but I need to add a custom label with 10F resp 10C next to the ProgressBar which results in a lot of extra code, as
the show_percentage option hardcodes a %, which does look odd when used for temperatures - using the builtin options sounds much better to me than a lot of boilerplate code that needs to be added.

Would it be possible to have the used character configurable?
Is that something the project would be open to?