Add basic line-following drivetrain

Works by drawing a Nth degree polynomial to guide us to the target and
guiding a lateral controller to follow it given a speed provided by the
driver.

Change-Id: I94fd31666769ca398b66f04665f39e01cee00627
diff --git a/frc971/control_loops/drivetrain/drivetrain.q b/frc971/control_loops/drivetrain/drivetrain.q
index 04bdaaf..66e9c32 100644
--- a/frc971/control_loops/drivetrain/drivetrain.q
+++ b/frc971/control_loops/drivetrain/drivetrain.q
@@ -81,7 +81,8 @@
     // Type of controller in charge of the drivetrain.
     //  0: polydrive
     //  1: motion profiled position drive (statespace)
-    //  2: spline follower.
+    //  2: spline follower
+    //  3: line follower (for guiding into a target)
     uint8_t controller_type;
 
     // Position goals for each drivetrain side (in meters) when the