Make the list of text extensions configurable

[vlsi]

Currently, no diff window is opened for .yaml and .yml extension which is surprising:

ApprovalTests.Java/approvaltests/src/main/java/org/approvaltests/reporters/GenericDiffReporter.java

Line 24 in 07fdbad

public static List<String> TEXT_FILE_EXTENSIONS = Arrays.asList(".txt", ".csv", ".htm", ".html", ".xml", ".eml",

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