Update plotting entrypoint with folder support

[jteijema]

No description provided.

[PeterLombaers]

Just to make clear what your intention is: You would like to be able to pass either a directory or a list of file paths to the asreview_files argument. And in case of a directory, it should plot all the .asreview files in that directory. It should not recursively go into subdirectories looking for .asreview files?

[PeterLombaers]

In #59 you moved the part that produces a list of files from the input to a separate function _get_files. That seems nicer.

[PeterLombaers]

Personally I always get a bit confused when reading list comprehensions with many parts. I really have to think hard to make sense of it. I would prefer something like

filepaths = []
for path in args.asreview_files:
    path = Path(path)
    ...
    filepaths.append(some_filepath)