Make apply operation happen out-of-place

[aaronlehmann]

The current implementation of continuity apply checks an existing directory against a Continuity manifest. A better model would involve specifying a source and destination, so that Continuity creates the target based on the manifest and data source. The data source could take multiple forms, such as a tar file, preexisting directory on disk, version control repository, etc.

Possible syntax:

continuity apply --source <dir|tar|etc.> <manifest> <target>

For the dir case, it would be more efficient to move files from the source directory instead of copying them. Perhaps this should be made optional, by splitting dir into copydir and movedir options. This could be further refined to support special features for copy-on-write filesystems such as btrfs.