Add support for Semaphore 2.0

[toastercup]

This adds support for the new count/index environment variables used in Semaphore 2.0's jobs (previously, "threads"), per https://docs.semaphoreci.com/article/50-pipeline-yaml#parallelism

@ArturT I've left CHANGELOG.md untouched - let me know if you'd like me to append to it prior to merge, or if that's something you'd rather handle.

Thanks!

[ArturT]

Thanks @toastercup for PR. It looks good. I will take care of changelog.

I will have to update also KnapsackPro/knapsack_pro-ruby#77 as back then when I added support for Semaphore CI 2.0 the SEMAPHORE_JOB_COUNT and SEMAPHORE_JOB_INDEX were not available.

I will try to merge this PR soon.

[ArturT]

BTW this is something you may find interesting how to split tests in a dynamic way with Queue Mode in knapsack_pro gem on Semaphore 2.0
https://docs.knapsackpro.com/2019/run-parallel-jobs-on-semaphore-ci-2-0-to-get-faster-ci-build-time

There is also example of how to use it with CodeClimate
https://docs.knapsackpro.com/2019/how-to-merge-codeclimate-reports-for-parallel-jobs-ci-nodes

[toastercup]

@ArturT That's quite nifty - thank you!

[ArturT]

I've released a new version of knapsack 1.18.0. :)

[ArturT]

I also updated each client for Knapsack Pro to support Semaphore 2.0 parallelism property and ENVs.
https://docs.knapsackpro.com/integration/

All examples for Ruby & JS were updated in article https://docs.knapsackpro.com/2019/run-parallel-jobs-on-semaphore-ci-2-0-to-get-faster-ci-build-time