Add nf-core style custom configs

[olgabot]

Adds more flexibility for specifying parameters: https://github.com/nf-core/rnaseq/blob/master/nextflow.config

czbiohub/sc2-illumina-pipeline pull request

Many thanks for contributing to czbiohub/sc2-illumina-pipeline!

Please fill in the appropriate checklist below (delete whatever is not relevant). These are the most common things requested on pull requests (PRs).

PR checklist

• [x] This comment contains a description of changes (with reason)
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [ ] If necessary, also make a PR on the czbiohub/sc2-illumina-pipeline branch on the nf-core/test-datasets repo
• [ ] Ensure the test suite passes (nextflow run . -profile test,docker).
• [ ] Make sure your code lints (nf-core lint .).
• [ ] Documentation in docs is updated
• [ ] CHANGELOG.md is updated
• [ ] README.md is updated

Learn more about contributing: CONTRIBUTING.md