Make apply operation happen out-of-place

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
```

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.