ENH: Add option to mne browse_raw

Needed for looking at raw data that has been processed with maxshield.

It would be nice if we could also filter the data when displaying it. Is this something people would be interested in? If so, I'll probably make it so that 4th-order IIR filtering can be done on chunks before they are shown, and add `--highpass` and `--lowpass` options to the script. Not sure what it will be like performance-wise, but it's probably worth checking. If people think this is a good idea, then don't bother merging this PR for now (I can just add to it).
Note that