pass context into the builder.

doc/airline.txt

@@ -300,6 +300,9 @@ statuslines to your liking.  Here is an example: >
     return -1
   endfunction
 <
+The second variable is the context, which is a dictionary containing various
+values such as whether the statusline is active or not, and the window number.
+
                                              *airline-pipeline-return-codes*
 The pipeline accepts various return codes and can be used to determine the
 next action.  The following are the supported codes: >