Add interactive simulation example

[vincekurtz]

Adds an example of running a mujoco simulation where the user can interact with the model while the controller is running.

Also updates the pendulum.xml model so it looks good and swings properly.

[alberthli]

Note: in its current form, there will be minor merge conflicts with #20, which also edits the pendulum.xml file to address the lack of actuator.

[alberthli]

Approved once you commit the one suggested change.