Flag known files in the viewer

[Rob--W]

Forking from https://github.com/Rob--W/crxviewer/issues/35#issuecomment-272752820 :

Sounds good. Maybe even with a list of known libraries *, or inputting an external URL to verify that the two files match.

* e.g. from https://github.com/mozilla/amo-validator/blob/master/validator/testcases/hashes-allowed.txt

Another thing to consider is trailing whitespace. I have seen too often that a hash does not match because of a difference in whitespace.