Allow a default subject to be provided

[irace]

As per @prendio – allow a default subject to be provided rather than explicitly registering subjects for individual activity types using the registerSubject:forActivityType: method added in this pull request.

(This isn’t particularly pressing since there isn’t public API for extensions to read this value, and only Mail and Messages support subjects out of the built-in activities (which only change once a year, at most), but we only just realized that the latter does yesterday, so who knows what else we could potentially be missing?)

A couple of options:

• Use the attributedTitle value if it exists and a subject was not explicitly registered for a given activity type
• Add a @property (nonatomic, copy) NSString *defaultSubject; property
• Document that calling registerSubject:forActivityType: with a nil activity type will register it as the default. I think I like this one the best.

Thoughts?

[prendio2]

registerSubject:forActivityType: with nil activity seems like a good call to me

[AriX]

Using attributedTitle as a fallback default also seems like a good idea.

[irace]

I think either would be good but this issue spelling out how complicated the API is becoming might be a compelling reason for favoring attributedTitle.

[irace]

Closing in favor of the refactor proposed in this issue: #43