Add Application Key to Pushover Integration#1071

Hey all -

The Pushover integration currently utilizes only a User Key.
Pushover incorporates two keys: User Key and Applicaiton Key. This is one of the strengths of Pushover.

The Application Key allows for notifications to be organized by source applicaitons, each with their own 72x72 pixel PNG icon. It makes notifications easy to identify visually and to be grouped by source in a list.

It would be great if an Applicaiton Key can be configured in the Pushover integration.