0.6.4: Add syntax highlight for Swift code between backticks

Swift is a general-purpose, multi-paradigm, compiled programming
language developed by Apple Inc. for iOS, macOS, watchOS, tvOS, and
Linux. Swift is designed to work with Apple's Cocoa and Cocoa Touch
frameworks and the large body of extant Objective-C (ObjC) code written
for Apple products. Swift is intended to be more resilient to erroneous
code ("safer") than Objective-C, and more concise. It is built with
the LLVM compiler framework included in Xcode 6 and later and, on
platforms other than Linux, uses the Objective-C runtime library,
which allows C, Objective-C, C++ and Swift code to run within one
program.