[Feature suggestion] Allow for relevant annotation selection during processing.

[gael-vanderlee]

Hello,
When using the pipeline, it would be useful for it to allow for relevant annotations or events that should be considered for channel rejection. Typically, the time between trial should be ignored, and I have annotations that encompass relevant time periods. More commonly, researchers use mne's epochs to find relevant portions of their data.
Rejecting channels outside of those time periods is irrelevant.

This joins #108. I'm not sure why PREP doesn't support discontinuities, but if that's a limitation that we can circumvent we definitely should.