add lr_finder

[cmcmaster1]

No description provided.

[dfalbel]

Thanks very much @cmcmaster1 !

I changed a little the API for plotting, instead of the plot = TRUE argument we now have an S3 method for the result of lr_finder. This way its easier to change the plot location and/or save it to a file.

Also added a small test case. I still want to add an example to the docs and then we are good to merge!

[cmcmaster1]

Yeah, I certainly wasn't sold on my implementation. This makes sense.