[commands] Add option to process a certain command_str

[Puncher1]

Summary

This adds the feature requested here: #7067
It's worth mentioning that this is not useful when processing one command, but processing multiple commands or a certain part of message.content.

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 is actually not particularly what was asked for. The original FR is actually trickier to implement and at this point mostly a wontfix.