Add environment image library support data.

[adamsilverstein]

In WordPress/phpunit-test-runner#134 we are adding the collection of environmental data about the GD/Imagick support of the tested environment.

We should display that data in the report once it is available, as suggested in this comment: WordPress/phpunit-test-runner#134 (comment). The two locations to consider are:
https://github.com/WordPress/phpunit-test-reporter/blob/master/parts/single-result.php#L50
https://github.com/WordPress/phpunit-test-reporter/blob/master/src/class-display.php#L337

[adamsilverstein]

@getsource when you have a chance, this is ready for review.