Clean up at_exit into separate methods

[colszowka]

The at_exit handling in simplecov/defaults.rb is getting a bit out of hand, especially so with the stuff added thanks to #151. This should probably be moved to separate methods similar the custom SimpleCov.at_exit is already handled.

[pmontrasio]

I'm trying to build a statistical tool based on the coverage values stored in .last_run.json
I'm using the at_exit hook but unfortunately an inspection of simplecov/defaults.rb shows that the file is written after at_exit is run. This means that I'm not getting the latests results and the timestamp of the file (the latter is less important because I have Time.now).

Being an at_exit method I assumed it was run after everything else. How about an after_all method that runs really at the end, after the at_exit block?

By the way, do you have any suggestion about how to run my code when simplecov stops running? Thanks.

[pmontrasio]

I discovered that I can read the data I need from SimpleCov.result from within the at_exit block. This solves my problem and gives me access to much more data.
Nevertheless it would be nice to have a real at_exit hook running after everything else.

[PragTob]

Fixed 💃