Update type definition for Signalflow client (#119)

Add `filter` param to the Signalflow client `ExecuteOptions`
definition (allows filters to be passed in separately from
the program text when executing Signalflow).

lib/signalfx.d.ts

@@ -14,6 +14,7 @@ export interface ExecuteOptions {
     usedByDetectorUI?: boolean;
     useCache?: boolean;
     compress?: boolean;
+    filter?: string;
 }
 
 type SignalFlow = (token: string, options: SignalFlowClientOptions) => SignalFlowClient;
@@ -270,4 +271,4 @@ export const CONSTANTS: {
     };
 };
 
-export * from './ingest';
+export * from './ingest';