Add option to save annotations as plain text

Currently, annotations can only be saved to a FIFF file. Since this is a format that can’t be read outside of MNE easily, an option to save annotations as plain text would be very useful. I can work on this if you think this should be added (e.g. as an optional argument to annotations.save, for example format="CSV" or kind=textortype=TXT - let me know what you prefer). @massich what do you think?

[larsoner]

What do you think of having a “lightweight” export option, meaning that if you export annotations as CSV only onset, duration, and descriptions are exported (a warning message could make this obvious to the user)?

I don’t see why we wouldn’t also include meas_date in there in a comment, like:

# MNE Annotations CSV
# orig_time: (X, Y)
# onset, duration, description
502.45682716, 0, BAD_EDGE
...

Then on read if it’s there, use it, if it’s not, it’s the same as constructing Annotations with orig_time=None

[jona-sassenhagen]

Closed for #5699