Improve highlighting of #available condition

Conforming to Xcode, the syntax file now highlights platforms contained
within the `#available` condition.

It also now removes highlighting numbers and the "*" (i.e, symbol for
future platforms) that are contained within the `#available` condition
since they are not literal numbers or operator (thanks to @acclissold
for the idea)

Additionally, add tests to example.swift to demonstrate highlighting of
them.

References #56

example/example.swift

@@ -266,3 +266,10 @@ let dict = [
     "fadsf",
   ],
 ]
+
+if #available(OSX 10.10.3, *) {
+    // Use APIs OS X 10.10.3 and onwards
+}
+if #available(watchOS 2, iOS 9.0, OSX 10.11, *) {
+    // APIs available to watchOS 2.0, iOS 9.0, OSX 10.11 and onwards
+}