refactor: move more commands to internal/cmd (#207)

* refactor: move more commands to internal/cmd

Part of https://github.com/ooni/probe/issues/1335.

We would like all commands to be at the same level of engine
rather than inside engine (now that we can do it).

* fix: update .gitignore

* refactor: also move jafar outside engine

* We should be good now?

internal/engine/httpheader/accept.go

@@ -0,0 +1,6 @@
+package httpheader
+
+// Accept returns the Accept header used for measuring.
+func Accept() string {
+	return "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
+}