Add target selection based on buttons + mode

This makes ball vs hatch mode discriminate among targets on the rocket,
and adds buttons to control which target on the cargo ship we go for.

Also, add new color to indicate that we have a target (purple flashing).

Change-Id: Ifc734c8168a1814511dea95abc361eb13383f597
diff --git a/y2019/BUILD b/y2019/BUILD
index f2150ed..c49e7c4 100644
--- a/y2019/BUILD
+++ b/y2019/BUILD
@@ -136,6 +136,7 @@
         "//frc971/control_loops/drivetrain:drivetrain_queue",
         "//frc971/control_loops/drivetrain:localizer_queue",
         "//y2019/control_loops/drivetrain:drivetrain_base",
+        "//y2019/control_loops/drivetrain:target_selector_queue",
         "//y2019/control_loops/superstructure:superstructure_queue",
         "@com_google_protobuf//:protobuf",
     ],