Frames within a pattern (longer patterns)

1 pattern = 4 frames
1 frame = 16 steps

Currently, it stays in the same first frame.
Rename `current_channel` variable for clarity
Advance current frame when all steps were played
Use arrows to move between frames
Reset current frame when stopping
Change steps in current *selected* frame
Print current (selected) frame at the bottom
'Follow cursor' toggle
Delete key clears currently selected frame
Disable Follow when Left or Right is pressed
Change set of step colors

Still, they are way too dark... I'll need to setup a custom palette soon.
Render a pattern map

Show steps and frames from all channels at once!
Merge pull request #10 from munshkr/frames

Frames