Add https://github.com/TartanLlama/optional

This gives a C++17-like optional which I want to use for camera code.

Change-Id: I1dc2f83040a5a76f50646280a6a1d11f29f8addc
diff --git a/third_party/optional/.clang-format b/third_party/optional/.clang-format
new file mode 100644
index 0000000..9b3aa8b
--- /dev/null
+++ b/third_party/optional/.clang-format
@@ -0,0 +1 @@
+BasedOnStyle: LLVM