Improve train status formatting

This change adds proper singular/plural labels for minutes and hours in the lateness display, fixes green status text contrast for dark mode, and updates the README to credit the Amtrak.js models used as a base.

Sources/AmtrakStatus/Helper.swift

@@ -0,0 +1,7 @@
+import Foundation
+
+extension Int {
+    func pluralized(singular: String, plural: String) -> String {
+        self == 1 ? singular : plural
+    }
+}