make mne-report bids aware

[jasmainak]

This is something me and @teonbrooks discussed before.

mne-report could make use of the bids structure for quality assurance. Here are some things that come to mind:

1. Evoked butterfly plots for different conditions
2. raw.plot with bad channels marked in red
3. plot_events
4. plot cov
5. Display meta information in sidecar files (events.tsv, channels.tsv and _meg.json)
6. Show the stimuli files (images for example)
7. Show fiducial locations

Feel free to edit this description to add more to the list

[hoechenberger]

To achieve this all we'd need to do is wrap parse_folder(). With the new tagging mechanism in MNE-Python 0.24, it would also be possible to organize and filter the generated content nicely.