Social: pushover; add callback parameter w/ documentation

[brianmangan]

• Bugfix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)

Proposed changes

Pushover offers a callback feature where a user can pass a url.
I have added the required variables, parameters and documentation. This value is only passed by msg.callback.

Checklist

• [ x] I have read the contribution guidelines
• [ x] For non-bugfix PRs, I have discussed this change on the forum/slack team.
• [ x] I have run grunt to verify the unit tests pass
• [ n/a] I have added suitable unit tests to cover the new/changed functionality

[dceejay]

looks good - you happy with it now ?

[brianmangan]

I am happy with it now, thank you!