Implement delegation of particular files in directories about python-tuf HOT 2 CLOSED

Really, #52 is the best way to do this. When signercli.py is able to read input from a file, it is easy to imagine such an input...

{"commands":[{"command":"makedelegation", "inputs":{"positional":[...], "keywords":{"targets":[...]}}}]}

...where the commands[0]["inputs"]["keywords"]["targets"] is used to specify targets on a file-by-file, instead of directory, basis.

from python-tuf.

Delegated roles are no longer restricted to a single directory; multiple and arbitrary paths may be specified:
https://github.com/theupdateframework/tuf/blob/master/tuf/repo/signerlib.py#L1038-L1051

Relevant commits:

Continue design changes to address issue #47
11958cd

Update following Trishank's initial code review
b5fcaae

from python-tuf.