Hashing consecutive items with plugins is corrupted

[ankostis]

Currently plugins install digester-instances when initialized.
The SAME-DIGESTERSs are re-used multiple times, corrupting hashes for all but the 1st input!

But in reality, nobody could have discovered this because plugins simply DID NOT WORK :-)

These bug existed since plugins were originally introduced(#9) so the culprit falls completely on me!
And it has not been discovered because TravisCI were not checking them.

To deal with these:

• I have already started installing TCs, and
• fixing the plugins (ankostis/omnihash@a21bf38)
• in order to provide a PR for the corruption bug.

[ankostis]

See this build for the problem: [edit] https://travis-ci.org/ankostis/omnihash/builds/184589768

Also see this for why when trying to work across PY-versions is easier if not shadowing package-name with module, and using relative imports instead:
https://travis-ci.org/ankostis/omnihash/jobs/184589769

[Miserlou]

Eagerly awaiting your PR..