More user friendly output from `filepack verify`

[mebious-dev]

Printing the manifest on successful verification doesn't provide much utility in communicating the output and can be extremely lengthy with larger directory trees.

A possible alternative is to print a summary listing the total number of hashed files, the number of verified files and (if verification has failed) a number of unverified files.

[casey]

--print is intended to both verify the manifest and print JSON so it can be used in a pipeline and guarantee that you're only using validated data:

$ filepack verify --print | some-other-function

That being said, I'd like to support more user friendly output for filepack verify, so I'll hijack this issue title to reflect that.

[mebious-dev]

In the meantime additional documentation to clarify the usage of --print would be ideal, whether it is documented in man pages or in external documentation.

[casey]

Makes sense! Added some docs in #79.