Support FailureDiffs without configuration

HaveValidSnapshot.swift

@@ -99,6 +99,8 @@ func getDefaultReferenceDirectory(_ sourceFileName: String) -> String {
     let folderPathComponents = pathComponents.subarray(with: NSRange(location: 0, length: currentIndex)) as NSArray
     let folderPath = folderPathComponents.componentsJoined(by: "/")
 
+    setDefaultFailureDiffsDirectory(testTargetFolderPath: folderPath)
+
     return folderPath + "/ReferenceImages"
 }
 
@@ -111,6 +113,10 @@ private func parseFilename(filename: String) -> String {
     return sanitizedName
 }
 
+func setDefaultFailureDiffsDirectory(testTargetFolderPath: String) {
+    setenv("IMAGE_DIFF_DIR", "\(testTargetFolderPath)/FailureDiffs", 0)
+}
+
 func sanitizedTestName(_ name: String?) -> String {
 	guard let testName = currentTestName() else {
 		fatalError("Test matchers must be called from inside a test block")

README.md

@@ -178,3 +178,14 @@ implement the `ViewResizer` protocol and resize yourself.
 The custom behavior can be used to record the views too.
 
 For more info on usage, check the [dynamic sizes tests](Bootstrap/BootstrapTests/DynamicSizeTests.swift).
+
+## A note about test failures artifacts
+
+By default Nimble-Snapshot will place test failures image diffs in a folder right next to `ReferenceImages`. It is strongly recommended that you don't commit those files to your source repo.
+
+Add this to your `.gitignore`:
+
+```
+# Nimble-Snapshot
+FailureDiffs
+```