Add delay kwarg to message.delete()

[Vexs]

Summary

This PR adds a delay kwarg to message.delete that delays the deletion of a message while yielding back to the event loop. This is the same behavior used in delete_after kwargs, but for more generic usage. It also refactors existing delete_after behaviors to use the new kwarg.

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]

Not sure I'm too keen on the wording. The kwarg is delete_after not delete_delay but await x.delete(after=30.0) is different from after used elsewhere in the lib (i.e. a callback). I suppose the naming itself is fine.

[Rapptz]

Suggested change

	async def delete(self, delay=None):
	async def delete(self, *, delay=None):

This needs to be a keyword only argument. await m.delete(10.0) is bad readability and could be ambiguous in the future.

Also, it needs an addition in the docstring.

.. versionadded:: 1.1.0
   The ``delay`` keyword-only parameter.

[Vexs]

Oh bugger I completely forgot. Yes, this should be that.