[Feature] Add API to define and run report custom converter

[jerome-benoit]

• report converter API: a callback as a run() option:

<T>(report: Report) => T

• report display converter API: display is done incrementally by the following primitives:

  benchmarkError
  benchmarkReport
  br
  groupHeader
  header
  size
  summary
  units
  warning

Exposing an API to override them all will result in a bloated one. The way report is displayed must be simplified first.