Adds basic support for the Swift programming language

Text only lexer for now until Pygments catches up

samples/Swift/section-23.swift

@@ -0,0 +1,11 @@
+let vegetable = "red pepper"
+switch vegetable {
+    case "celery":
+        let vegetableComment = "Add some raisins and make ants on a log."
+    case "cucumber", "watercress":
+        let vegetableComment = "That would make a good tea sandwich."
+    case let x where x.hasSuffix("pepper"):
+        let vegetableComment = "Is it a spicy \(x)?"
+    default:
+        let vegetableComment = "Everything tastes good in soup."
+}