adding selectPlugin API to plugins

Summary:
Allow linking from one plugin to another. Adds a prop `selectPlugin` to the plugin that can be passed a pluginID and an optional `deepLinkPayload`.

The return value tells you if switching the plugin was successful.

```
selectPlugin: (pluginID: string, deepLinkPayload: ?string) => boolean,
```

Reviewed By: passy

Differential Revision: D10483925

fbshipit-source-id: 6f821277150b2db185b7d545c310214a11432eac

src/plugin.js

@@ -40,6 +40,7 @@ export type Props<T> = {
   setPersistedState: (state: $Shape<T>) => void,
   target: PluginTarget,
   deepLinkPayload: ?string,
+  selectPlugin: (pluginID: string, deepLinkPayload: ?string) => boolean,
 };
 
 export class FlipperBasePlugin<