Write sample playback code

[catblade]

Single track sample playback code included
Need to expand to multi-track, lots of room for further improvements

[skafreak]

Initial cut at multi-track playback in d1638a3
Ongoing improvements:

• Issue reading files from SD card, need to determine if this can be fixed in code or if there is a read speed limitation
• Need user input for programming new sequences
• Possible issue with not all tracks playing on first step (need to investigate further)
• Add stepping LEDs to show current step
• General improvements to UX (increase responsiveness, start time, etc.)