Adds basic support for the Swift programming language

Text only lexer for now until Pygments catches up

samples/Swift/section-21.swift

@@ -0,0 +1,8 @@
+var optionalString: String? = "Hello"
+optionalString == nil
+
+var optionalName: String? = "John Appleseed"
+var greeting = "Hello!"
+if let name = optionalName {
+    greeting = "Hello, \(name)"
+}