Add support for string labels

[AlexanderZender]

I got some datasets which have labels that are string based instead of int. Currently, when these are passed down to explainer dashboard it will break, as the target is converted to a float values. I convert these labels using a label encoder, but this occurs in my model.

It would be a great addition if you could support string labels, it would give more readability to the dashboard with such datasets. As the user would not have to know which value was cast to what in the label encoder, and would also allow the support for label encoding in models/pipeline.