Allow downloading XIP without installing

[interstateone]

There should be a --no-install flag that also requires a --destination option (see #100) (since keeping the XIP in application support and moving it to /Applications are both wrong for this use case), which downloads the XIP and then finishes without installing it. This would be useful for caching purposes.

If using aria2 to download, download the XIP directly to the destination directory instead of to application support only to move it.