add reason kwarg to thread delete method

[owocado]

Summary

Adds the missing reason keyword-only argument to delete method for discord.Thread instances. As per discord API docs, deleting the channel endpoint already supports X-Audit-Log-Reason header.

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]

Thanks.