Implement mas reload

[rgoldberg]

Reload Page on certain pages in the Mac App Store updates some of the Apple data structures used by mas.

Add sub command (tentatively called mas reload, to use the same terminology as the Mac App Store, though I'm willing to change the name, or to make an alias, to something that reads better, like mas refresh) to perform the equivalent of reloading the purchased list, the updates list, potentially other pages from the Mac App Store, or some combination thereof (including all).

Could accept flags specifying which things to reload. Default could be all of them, or that could be specified via an --all flag.

Also, this same functionality could be called by other commands behind the scenes to ensure they work with the latest data.

mas reset doesn't accomplish what Reload Page does in the Mac App Store, as per #632 (comment).