Expose basic stats to option `template`

[GregoryAlbouy]

Description

See #82 for context.

Once a benchmark done, we display a default summary featuring some basic stats (min, max, mean durations).
This summary can be overridden by a custom template with access to all output.Report data.
However it is currently not possible to access the computed basic stats from the template.
We want to address that by exposing basic stats to config option template.

Tasks

• Expose basic stats (min, max, mean durations) to option template
• Make sure no computing is done if the stats are not requested
• Make sure the computing is done only once if stats are requested several times

Suggestions

Notes

• Part of #82