Custom object exporters in Dumper

[JanTvrdik]

• bug report? no
• feature request? yes

I'm missing functionally similar to Tracy's Dumper $objectExporters.

Currently there is one hard-coded exporter for instances of DateTimeInterface. It would be nice to add support for custom exporters.

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Motivation:

I would like to see

Failed: [App\Model\MyEntity#1, App\Model\MyEntity#2, App\Model\MyEntity#3] should be
    ... [App\Model\MyEntity#2, App\Model\MyEntity#3, App\Model\MyEntity#4] in testSomething()

instead of

Failed: [App\Model\MyEntity(#017b), App\Model\MyEntity(#1a98), App\Model\MyEntity(#85c2), ...] should be
    ... [App\Model\MyEntity(#1a98), App\Model\MyEntity(#85c2), App\Model\MyEntity(#a014)] in testSomething()