Option for `deno bench` to output markdown table

[boyeln]

It would be very convenient if the deno bench could output a markdown table. My use case is to post the result in a github comment on PRs, but I belive this would be convenient many places. Like just copying the results into a readme file, or any other place that supports markdown.

Currently the default output of the deno bench is very close to a markdown table, but not quite. Not sure if the best approach is to change the default output, or just add a --markdown option (probably the latter since people might depend on the current format).

[ry]

Might as well default to markdown. PRs welcome!