Support strings for marker descriptions

[cbrnr]

Currently, we only support integer marker descriptions for "Stimulus" and "Response" marker types. String descriptions are only possible with "Comment" types.

This is a serious restriction, because it can lead to loss of information. I suggest that we also allow string descriptions for "Stimulus" and "Response" markers, which would allow us to just use the original (e.g. mne.Annotations) description.

Note that the specs do not contain our restriction at all. I already discussed this with my contact at BrainProducts, he says that this is a convention originated from their recorder software (which uses individual trigger lines). The format itself does not mandate integer marker descriptions at all (which was already mentioned here).