[gui] Add gif compression option

[houkensjtu]

Currently, if a user export the results of a simulation as a gif animation according to the method described in the documentation, most likely the size of the file will be tens of MB, which is certainly too heavy to upload to any web page. Since it's quite common for a user to use the resulting animation on a website, can we provide a simple compression option in the ti gif command to help the user to reduce the size of the gif file?

[neozhaoliang]

I would suggest making the command ti gif and ti video into one command

ti video script gui_name [options]

For example, if I want to make a video of the animation in mpm88.py displayed in the GUI mpm_gui, I can run

ti video mpm88.py mpm_gui -framerate 30 -quality 15 ... result.gif 

This should be easy to implement but will require some third-party modules like imageio or moviepy.

[ailzhang]

Closing via #6289