Feat: consume messages per partition

Pull request checklist:

[x] Include new test cases (either end-to-end or unit tests) with your change.

Existing test cases do not include consuming via the standard API. So did not make those for this case.
[x] Follow our style guides.
[x] Make sure all tests are still passing and the linter does not report any issues.

Current master has only quite a few linter errors. I made sure to not add any new errors and I've fixed a few.
[x] End files with a new line.
[x] Document the new code in the comments (if it is JavaScript) so the documentation generator can update the reference documentation.
[x] Avoid platform-dependent code.
[x] Note: If making modifications to the underlying C++, please use built-in precompiler directives to detect such platform specificities. Use Nan whenever possible to abstract node/v8 version incompatibility.
[x] Make sure your branch is up to date and rebased.
[x] Squash extraneous commits unless their history truly adds value to the library.

Blizzard / node-rdkafka