[ENH] Playback plugin

[gabrielbmotta]

With all the talk of playback in today's meeting, I figured I'd try to add playback from a file, mostly for testing purposes.

Currently supports:

• Streaming from a fiff file.
• Includes digitizer info.
• Fixed buffer size and rate of receiving data: equal to sample rate once per second

Potential future features:

• Adjustable buffer size
• Adjustable playback rate