tools: draw-importance: Introduce --csv to export as csv
tools: draw-importance: Introduce --csv to export as csv

Right now, when we do large processing of data, we are not storing
the mean importance and frame type anywhere, so we require to save
it to a file, by having an export option to csv, this makes things
easier.

To use: append --csv option along with other modes.
This is only applicable to batch mode

xiph/rav1e,xiph/rav1e