Comparison outputs should be colorful when pasted into GitHub

Copying and pasting a markdown comparison report to GitHub will render as tables and look like this:

Sometimes, pasting as preformatted text is denser:

|  T{ct}  |  OffsetT{ct}  |  Elements{io}  |   Ref Time |   Ref Noise |   Cmp Time |   Cmp Noise |      Diff |   %Diff |  Status  |
|---------|---------------|----------------|------------|-------------|------------|-------------|-----------|---------|----------|
|   I8    |      I32      |      2^16      |   6.092 us |       7.97% |   6.189 us |       9.24% |  0.097 us |   1.60% |   SAME   |
|   I8    |      I32      |      2^20      |   6.703 us |      11.35% |   7.095 us |      12.43% |  0.391 us |   5.84% |   SAME   |
|   I8    |      I32      |      2^24      |  14.092 us |       2.18% |  16.161 us |       2.11% |  2.068 us |  14.68% |   SLOW   |
|   I8    |      I32      |      2^28      | 115.971 us |       0.72% | 128.828 us |       0.24% | 12.857 us |  11.09% |   SLOW   |

(a bit wider but less long)

Both versions loose color though, which makes reading the benchmarks on GitHub harder. The console output contained colors. We should find a way to make GitHub render the comparison output with colors as well. One way to do this, is to abuse the diff highlighting and prepend lines with +, -, etc.:

 |  T{ct}  |  OffsetT{ct}  |  Elements{io}  |   Ref Time |   Ref Noise |   Cmp Time |   Cmp Noise |      Diff |   %Diff |  Status  |
 |---------|---------------|----------------|------------|-------------|------------|-------------|-----------|---------|----------|
 |   I8    |      I32      |      2^16      |   6.092 us |       7.97% |   6.189 us |       9.24% |  0.097 us |   1.60% |   SAME   |
!|   I8    |      I32      |      2^20      |   6.703 us |      11.35% |   7.095 us |      12.43% |  0.391 us |   5.84% |   SAME   |
-|   I8    |      I32      |      2^24      |  14.092 us |       2.18% |  16.161 us |       2.11% |  2.068 us |  14.68% |   SLOW   |
+|   I8    |      I32      |      2^28      | 115.971 us |       0.72% | 128.828 us |       0.24% | 12.857 us |  11.09% |   SLOW   |

Kudos to @miscco for the idea.

No file or test is named in the issue. Start by tracing the comparison-report generation and its existing console color handling, then check how pasted output is formatted for GitHub. Done means the comparison remains readable when pasted into GitHub and conveys the status colors described in the issue.