Allow (optional) bundleID instead of (required) AppID

[irlabs]

An app's bundleID can be used, just as the appID to get App Store information about it.

It is useful to also have this feature in Siren, because it will be easier for the developer to get the bundleIdentifier instead of the appID (especially when the app hasn't been approved and released yet).

One more thing to mention: I chose for the bundleID to be a variable which needs to be set before a version check is performed. Alternatively, we could get the bundleID automatically from the mainBundle, just as is done with the appName. However I like to have more control over which bundleID is used, as it might differ between AppStore version and debug version.

(Also I added a specialized error for when it won't check the version, because the last version check was too recent)

[ArtSabintsev]

The bundle ID can be directly pulled out from info.plist, so I could (technically) remove the publicly exposed variable all-together, no?

Also, nice trick with default implementations of extensions leading to optionality.

[irlabs]

Sure, in theory you don't need both the public properties appID nor bundleID. But like I described, sometimes you might want control over which bundle ID to use. (Like when e.g. the development version has a different bundle ID, but still want to link to the App Store version.)

[ArtSabintsev]

Seems like a pretty small edge case (IMHO). I'll take a look at this after work and cherry-pick the changes I like.

I'm leaning towards the removal of the exposed appID all together.

[ArtSabintsev]

Unfortunately, this isn't a complete solution. The appID, also known as the trackId in the JSON that is returned needs to now be extracted so that when the user launches the app in the app store, it knows which app to launch.

I'll take care of that change soon - probably not tonight.

[ArtSabintsev]

Will merge for credit and overwrite with #56 after #56 is reviewed by @irlabs