Add option to store output of best trial so far

[hbredin]

Dump the output of the pipeline after each better trial, here:

pyannote-pipeline/pyannote/pipeline/experiment.py

Lines 296 to 301 in afed9a6

	if direction * loss < direction * best_loss:
	best_params = status["params"]
	best_loss = loss
	self.pipeline_.dump_params(
	params_yml, params=best_params, loss=best_loss
	)