Add --json to log_cat to print fully valid json

This makes it easy to use JSON based tooling to dig inside the results.

Change-Id: I483869fe27e117bdf86b9f775371842477ef1cbe
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/json_to_flatbuffer.cc b/aos/json_to_flatbuffer.cc
index 11ea3a2..6d4a2f1 100644
--- a/aos/json_to_flatbuffer.cc
+++ b/aos/json_to_flatbuffer.cc
@@ -840,7 +840,7 @@
     }
     if (size > max_vector_size_) {
       ++skip_levels_;
-      to_string_.s += "[ ... " + std::to_string(size) + " elements ... ]";
+      to_string_.s += "[ \"... " + std::to_string(size) + " elements ...\" ]";
       return;
     }
     to_string_.StartVector(size);