Add custom IMPORT_USER_MODULES to user_config.py

[abefetterman]

Modules listed in IMPORT_USER_MODULES will be imported before making gym environments, for example add 'pybullet_envs' to enable pybullet environments to be used in command line

[jachiam]

Hi @abefetterman! I just looked through---mostly looks great! But I'm curious, why does the custom import happen separately across so many files? Since, if I'm remembering correctly, the aim for the custom import modules is to allow gym registration, I think you only would need to put this in the __init__.py for spinup. Then the new Gym envs will be accessible from any other file that imports spinup and calls gym.make. (Please feel free to correct me if I'm wrong, I might be.)

[abefetterman]

I usually avoid putting external imports into __init__.py, as it can create unnecessary dependencies and make debugging difficult. However in this case it doesn't import anything by default, and can be toggled easily in the user_config, so I think we can get away with it. It does lead to a much cleaner implementation. See the updated code for the PR.

[pkol]

@abefetterman , @jachiam You may be also interested in my PR which may be better solution to this problem: #201