Migration to cucumber 2.0.0

Cucumber 2.0.0 introduces a new way of running specifications. Instead of relying on `testthat` or `devtools` test commands to run the tests, cucumber now owns the test execution process.

Follow the steps below to migrate your cucumber tests to the new version.

# 1. Remove `test-cucumber.R` file.

In the previous versions, you had to create a `test-cucumber.R` file with `cucumber::test()` call to run the tests.

Now, you can run the tests directly from the console or from the R script using the same `cucumber::test()` function. It now has a new interface and behavior that very similar to `devtools::test` and `testthat::test_package` as it now also calls `testthat::test_dir` internally.

# 2. Move steps definitions to `setup-*.R` files.

`cucumber::test()` no longer is responsible for loading step definitions.

Instead it is recommended to store step definitions in `setup-*.R` files in tests directory, alongside `.feature` files.

Convert your directory structure from:

    tests/
    ├── acceptance/
    │   ├── steps/
    │   │   ├── steps1.R
    │   │   ├── steps2.R
    │   ├── feature1.feature
    │   ├── feature2.feature

to

    tests/
    ├── acceptance/
    │   ├── feature1.feature
    │   ├── feature2.feature
    │   ├── setup-steps1.R
    │   ├── setup-steps2.R

All `test-*.R` files that are in the directory will be ignored by `cucumber::test()`. To run them use another call to `testthat::test_dir()`.