Add in diffToolSuffix to support ImageMagick cli

Author: Minho Yi

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -4,6 +4,7 @@ import XCTest
 ///
 ///     diffTool = "ksdiff"
 public var diffTool: String? = nil
+public var diffToolSuffix: String = ""
 
 /// Whether or not to record all new references.
 public var isRecording = false
@@ -299,7 +300,7 @@ public func verifySnapshot<Value, Format>(
       }
 
       let diffMessage = diffTool
-        .map { "\($0) \"\(snapshotFileUrl.path)\" \"\(failedSnapshotFileUrl.path)\"" }
+        .map { "\($0) \"\(snapshotFileUrl.path)\" \"\(failedSnapshotFileUrl.path)\" \(diffToolSuffix)" }
         ?? """
         @\(minus)
         "\(snapshotFileUrl.path)"
@@ -309,6 +310,10 @@ public func verifySnapshot<Value, Format>(
         To configure output for a custom diff tool, like Kaleidoscope:
 
             SnapshotTesting.diffTool = "ksdiff"
+
+        or ImageMagick:
+            SnapshotTesting.diffTool = "magick compare"
+            SnapshotTesting.diffToolSuffix = "diff.png && open diff.png"
         """
 
       let failureMessage: String