expose threshold for compare endpoint

[boshek]

Here:

conbenchcoms/R/compare.R

Lines 10 to 25 in 90b46d7

	compare <- function(type = c("runs", "benchmarks", "batches", "commits"),
	baseline,
	contender,
	...) {
	type <- match.arg(type)
	req <- req_url_path_append(
	conbench_request(),
	"compare",
	type,
	paste0(baseline, "...", contender))
	resp <- conbench_perform(req)
	resp_body_json(resp, ...)
	}

we should the ability to change these:

https://github.com/conbench/conbench/blob/fb80afebb798dff6608f6b31273501152254c442/conbench/api/compare.py#L136-L137

leaving the defaults alone.