[JUnit] Add JUnit Platform @Cucumber annotation

[mpkorstanje]

Maven Surefire and Gradle do not yet support discovery of non-class
based tests[1][2]. As a workaround you can either use the @Cucumber
annotation or the JUnit Platform Console Launcher.

Cucumber will scan the package of a class annotated with @Cucumber
for feature files.

package com.example.app;

import io.cucumber.junit.platform.engine.Cucumber;

@Cucumber
public class RunCucumberTest {
}

1. JUnit 5: Allow resource-based testing gradle/gradle#4773
2. https://issues.apache.org/jira/browse/SUREFIRE-1724

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage increased (+0.3%) to 87.571% when pulling b044364 on junit-platform-cucumber-annotation into 3dae98a on master.