Make the list of text extensions configurable

[vlsi]

Currently, no diff window is opened for .yaml and .yml extension which is surprising: https://github.com/approvals/ApprovalTests.Java/blob/07fdbad02cbcd678d818f94c25500021f34c37f4/approvaltests/src/main/java/org/approvaltests/reporters/GenericDiffReporter.java#L24

It would be nice if new extensions could be added to the list without releasing ApprovalTests

[frederikb]

I agree that configuration would help a lot! Could we in the interim add a few additional common text file formats to this list? I've created a PR for this: #423