feat(miniooni): make CLI much more user friendly (#913)

Part of https://github.com/ooni/probe/issues/2184, because I wanted
to allow swapping commands and options more freely.

As a side effect, this PR closes https://github.com/ooni/probe/issues/2248.

AFAICT, every usage that was legal before is still legal. What has
changed seems the freedom to swap commands and options and a much
better help that lists the available options.

internal/registry/allexperiments.go

@@ -1,11 +1,11 @@
 package registry
 
 // Where we register all the available experiments.
-var allexperiments = map[string]*Factory{}
+var AllExperiments = map[string]*Factory{}
 
 // ExperimentNames returns the name of all experiments
 func ExperimentNames() (names []string) {
-	for key := range allexperiments {
+	for key := range AllExperiments {
 		names = append(names, key)
 	}
 	return