Add support for Bitbucket Pipelines

[javierav]

No description provided.

[ArturT]

@javierav Thanks for PR. I will review it.

Notes:

Docs for envs:
https://support.atlassian.com/bitbucket-cloud/docs/variables-and-secrets/

env	description
BITBUCKET_PARALLEL_STEP	Zero-based index of the current step in the group, for example: 0, 1, 2, …Not available outside a parallel step.
BITBUCKET_PARALLEL_STEP_COUNT	Total number of steps in the group, for example: 5.Not available outside a parallel step.

[ArturT]

I've released a new version: knapsack 1.19.0

BTW You may want to learn more about dynamic test suite split (Queue Mode) and how this can help you save more time: https://docs.knapsackpro.com/2020/how-to-speed-up-ruby-and-javascript-tests-with-ci-parallelisation

How to split slow RSpec test files by test examples (by individual it)?
https://knapsackpro.com/faq/question/how-to-split-slow-rspec-test-files-by-test-examples-by-individual-it