Get generated fastdummies variables

[latot]

Hi all, something very useful would be, in some way, be able to get the generated variables by the function fastDummies, in some analysis we need to know the new variables, it is possible to get them with some regex + the old vars, but I think would be better to have a function here that handles that.

Thx!

[jacobkap]

Thanks for the suggestion. If you could make a PR for this feature that'd be great. Otherwise please ping me in a few weeks if I haven't implemented this yet.