Perf report: added custom --regressions map

Example for 4 performance samples (`a`, `b`, `c`, `d`):
1) Standard regressions are useful to compare implementations with single base (`a` - reference column).
Generated regressions columns are: `b vs a`, `c vs a`, `d vs a`
2) `--progress` regressions are useful for time-based progress tracking (a - Jan, b - Feb, C - Mar, d - Apr).
Generated regressions columns are: `b vs previous`, `c vs previous`, `d vs previous`
3) For example, there are two platforms `a,b` and two implementations: `a1, a2, b1, b2`. We want to show implementation update improvements. And we want to show how platform `a` is better than `b` (green=better, red=worse).
Generated regressions map with `--regressions=1-0,r2-0,r3-1` is: `a2 vs a1`, `a1 vs b1`, `a2 vs b2` with `red-green` coloring on `a2, b1, b2` columns. `b2` is green colored if `a2` is better than `b2` (because "reverse" flag is used here).

P.S. Standard regressions are equal to `--regressions=1-0,2-0,3-0,..,i-0,..`