Make cleanup delay configurable

[vE5li]

Currently oil buffers will always stay open until 2 seconds after they are not visible anymore, but that behavior can be problematic if you want to go back to the previous file using Ctrl+O.

I found myself constantly jumping back to the oil buffer instead of the previous file on accident, similarly some people might like this delay to be longer because they want to jump back to the oil buffer.

This PR aims to make the delay configurable. A delay of zero will cause the buffers to close instantly, resulting in behavior closer to telescope, while any other number will be taken as the delay in milliseconds. The default remains 2 seconds, so this is a backwards compatible change.

[stevearc]

This is a great idea and honestly something I should have done a while ago. Just a couple small requests about phrasing and behavior

[stevearc]

Looks great, thanks for sticking with it through the iterations!