ENH: add 'raw' attribute to annotation formats?

[NickleDave]

in some cases it might be useful to have the "raw" annotations in whatever way they were loaded, e.g. a dict or some other data structure besides the attrs attributes declared by the class

could have a raw attribute for all formats that just keeps that structure?

trade-off is this increases size -- wouldn't want it to be default

add a keep_raw argument to from_file that defaults to False?