Allow to specify custom software name and version (#94)

In turn, this allows us to identify as ooniprobe-cli v3.0.0-rc.5.

Closes #41.

ooni_test.go

@@ -15,7 +15,9 @@ func TestInit(t *testing.T) {
 	defer os.RemoveAll(ooniHome)
 
 	ctx := NewContext("", ooniHome)
-	if err := ctx.Init(); err != nil {
+	swName := "ooniprobe-cli-tests"
+	swVersion := "3.0.0-alpha"
+	if err := ctx.Init(swName, swVersion); err != nil {
 		t.Error(err)
 		t.Fatal("failed to init the context")
 	}