Feature request: Add JUnit 5 extension that invokes start/validateSnapshots

[grimsa]

Currently the following block has to be duplicated in every test.

    @BeforeAll
    static void beforeAll() {
        SnapshotMatcher.start();
    }

    @AfterAll
    static void afterAll() {
        SnapshotMatcher.validateSnapshots();
    }

It should be possible to add a simple JUnit 5 extension that would handle this.

Note for implementation:
JUnit 5 compile dependency should be optional - i.e. projects using JUnit 4 or an entirely different testing framework should not get transitive JUnit 5 jars.