tiny-count: shift parameters for overlap selectors

[AlexTate]

Overlap selector definitions have been extended to optionally accept 2 numerical parameters that shift the 5' and 3' ends of matching feature intervals. Alignments are then evaluated using this shifted interval in Stage 2 selection. They have the following syntax:

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
• If either parameter is provided, the other must also be provided
• Zero is also an accepted shift value

Shifting might produce an illegal interval. When this happens, the offending match (the feature-rule pair) is omitted from Stage 2 and 3 selection, but the feature's other matches remain. Illegal intervals include:

• Zero-length intervals (start == end)
• Inverted intervals (start > end)
• Negative intervals (start < 0)

Closes #278

[taimontgomery]

Tested with zswim8 and ram1 data. Needs more extensive testing once the next round of updates to feature selections are implemented.