Add exposed.headers configuration for CorsFilter

[RodrigoQuesadaDev]

Hi, I'm currently using Spring Boot and I would like to request for a way to configure the Access-Control-Expose-Headers value set when responding to CORS requests. This is required, for example, to be able to access custom response headers coming from CORS responses.

The new configuration path could be like this (following the convention you use here): stormpath.web.cors.exposed.headers

Then you just need to set the exposed headers on this class: https://github.com/stormpath/stormpath-sdk-java/blob/3134dde0ae1b5f944585e64e44b376a427c2ddc2/extensions/spring/stormpath-spring-webmvc/src/main/java/com/stormpath/spring/config/AbstractStormpathWebMvcConfiguration.java#L1577