[Feature] Add the number of killed enemies to info object

[ganyariya]

Description

This PR add the number of killed enemies into info object. For example, mario kills 3 goomba, then info["kills"] = 3.

Pros and Cons

• Pros
  • This feature can keep track of the number of killed enemies, for analyze log or play.
• Cons
  • To keep track of this, use 5 length list, so speed will be a very little slower.

Notes

This PR is based on #103 .

This PR is not need and not important for this gym-super-mario-bros ! So, If you feel that this PR is not important for gym-super-mario-bros, please close this :)

Type of change

Please select all relevant options:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] MGMT (non-breaking change to deployment, CI, etc.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works

[Kautenja]

Thanks for the PR! Unfortunately, this seems to break backwards compatibility with all versions of Python other than 3.9 (https://app.travis-ci.com/github/Kautenja/gym-super-mario-bros/builds/236973112). Because support of python 3.5-3.9 is necessary for this project, this PR cannot be merged at this time. I'm happy to consider the killed enemies feature if you put together a new PR without the type hints.