decimal_to_percent doesn't take nans

[christophermclement]

Using formatter decimal_to_percent doesn't allow nans. Ex. if I have a column showing an average and for some row there's simply no data it would have numpy.nan, the text formatter throws an error.

Perhaps it should just kick back numpy.nan or something similar for any nan or nan-like value, or even for any non-numeric (strings, etc).