New options for multi_scan!

• Need to tweak the options that are passed to scan just before a test run starts? Use the override_scan_options_block option to get called back and modify the options.
• Do you want to send a special command to your cloned iOS simulators just before they're used? Use the simulator_started_callback to get called back with the simulators's udid to perform any xcrun simctl call you want.
• Do you want to re-use your simulators for parallel iOS test runs? Pass in the reuse_simulators_for_parallel_testruns option set to true.

Cloned simulators also get their hardware keyboard disconnected like regular scan does for its simulators.