tiny-count: overlap selector shift parameters

[AlexTate]

Overlap selector definitions will be extended to optionally accept 2 numerical parameters. These will allow users to create rules that shift the 5'/3' ends of matching features by the specified number of bases.

selector, M, N
  M = shift value for 5' end
  N = shift value for 3' end

• Positive values shift the specified end in the 3' direction
• Negative values shift the specified end in the 5' direction