Add imageio as dependency in setup.py

[GiulioRomualdi]

Hi! First of all, thank you for implementing this package!

I tried to install it by following the documentation in the readme (i.e. pip install mujoco-python-viewer)
However, when I try to import mujoco_viewer I get the following error

Python 3.8.10 (default, Mar 15 2022, 12:22:08) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mujoco
>>> import mujoco_viewer
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/gromualdi/venv/test-mujoco/lib/python3.8/site-packages/mujoco_viewer/__init__.py", line 1, in <module>
    from mujoco_viewer.mujoco_viewer import MujocoViewer
  File "/home/gromualdi/venv/test-mujoco/lib/python3.8/site-packages/mujoco_viewer/mujoco_viewer.py", line 7, in <module>
    import imageio
ModuleNotFoundError: No module named 'imageio'

This PR adds imageio as a dependency in the setup.py

[rohanpsingh]

Ah, I missed that!
Thanks for the PR.