Add new flag and functionality to support checksum digest

[jordanpadams]

An attempt at #4 (adds --digestdir command line option in order to use pre-computed digests)

There are a number of assumptions here that'll probably need revisiting:

• Issue #4 says to add a true/false flag for checksum manifest; skipping that since it's implied by providing a flag for the manifest base directory
• Since we're providing a directory and not a single file, we're assuming that the directory contains a tree of possible manifest files
• Assuming manifest files are named manifest.tsv; all of them are read into memory
• Treating them as tab separated values of "filename" TAB "digest" (checksum)
• They're actually whitespace separated values
• No commenting or other syntax specified for them

The docs/samples directory now includes a sample tree of manifest.tsv files.

NOTE: These changes were rebased in internal repo so functionality will most likely need to be modified and tested.

[jordanpadams]

Closing as wontfix. This PR has been opened forever and this ship has sailed.