[commands] Added optional arguments to load_extension

[FrostByte266]

Summary

I have added the ability to pass extra positional and keyword arguments to an extension's setup method to provide a nice way to share data with an extension and the main bot code. I have tested this and code that doesn't pass extra arguments works the same as before, and passing positional and keyword arguments to the setup function also works.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

This isn't really a design space I want to explore like this. Indiscriminately passing parameters like this tends to block the load_extension from being changed (e.g. if I wanted to pass a package path to importlib in the future). There is another method being explored to allow to pass configuration in an extension but that'll be explored once the library moves on to Python 3.6.