Allow input of a file specifying filepaths to hash as input; produce dry-run output

[thompsonmj]

If a user has a list of filepaths that they need checksums for rather than simply a root directory, they should be able to pass a CSV with these instead of a root directory to walk.

In this case, hash exactly all of the files specified by this input (no more or less).

[thompsonmj]

On the other side of the same coin, create a dry-run option that outputs only the filepaths and filenames to be hashed. If saved as a CSV, this would then be suitable to use as input for what to hash.