Proposal: avoid using #at_exit to run the suite where possible

TL;DR: This patch moves spec running out of ruby's `#at_exit` handler except when autorunning is explicitly enabled.

The specs are currently run like so:
  - rspec is required
  - the specs themselves are required
  - the ruby process exits, triggering `#at_exit` callbacks
  - [this callback](https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/runner.rb#L8-L19) fires, which does the actual spec running.

The fact that the suite actually runs as the ruby process is exiting has caused some troubles for me recently.
  - A regular `#exit` call in unrelated code causes rspec to run the suite a second time.
  - Using `#exit!` to avoid that behaviour [as libraries like test-queue do](https://github.com/tmm1/test-queue/blob/master/lib/test_queue/runner.rb#L196) skips all the other `#at_exit` hooks, which skips writing coverage results, shutting down selenium browsers, and so on.

The run-during-exit behaviour has been present ever since rspec was forked from micronaut. David Chelimsky explained his rationale for it [here](https://www.ruby-forum.com/topic/212722) -- it makes running via both `ruby` and `rspec` easy:

When running via `ruby`, autorun is required, because all that happens is that rspec and the spec mentioned are loaded; the run isn't explicitly triggered. A `require spec/autorun` would be required in spec_helper.rb in this case.

    bundle exec ruby -Ispec spec/blah_spec.rb

But, in the common case that is used these days (often triggered via rake), there's no need for autorun, since we're running an rspec binary and we can trigger the run ourselves (as seen in my patch, in exe/rspec).

    rspec spec/blah_spec.rb

Anecdotally, I don't think the `ruby` case is ever really used these days. In any case, it'd be simple to detect when rspec was invoked by `ruby` and log about it if autorun wasn't required explicitly. If there's interest I'll add this.

This `#at_exit` use and the associated workarounds have caused other problems in the past, like this one:
https://github.com/rspec/rspec-core/pull/410

Anyhow, interested to hear everyone's thoughts on this.