Add unittests to slack annotations

[mtomilov]

Fixes #6

[seanh]

👍 I think this is going in a great direction, and I'm actually relieved to see that you've been able to refactor the one giant method into something reasonable :phew:

Just a couple of requests:

1. I think you should move conditional logic out of notify() and into the helper functions that it calls instead. See below for specific suggestion.
2. Move the Slack-formatting functions into a separate file
3. Group the tests for each function together into a test class
4. For patching use fixtures with autouse=True and autospec=True (see below for a specific suggestion)

[seanh]

There's quite a few separate functions to be tested, so it's probably worth group the tests for each function into a class. Just makes it easier to see where the tests for one function end and the next begins, and also opens up the possibility to localize fixtures into the classes that use them:

class TestGetSearchAfter:
    def test_without_cache(self):
        ...

    ...

class TestNotify:
    ...

class TestFoo:
    ...

...

[seanh]

I'd recommend doing this in a separate PR, but if we changed cli.py to pass an opened file object into notify() instead of a cache_path string, then we could add a unit test for get_search_after() that passes in a StringIO and properly tests how it reads the search_after value from the JSON file contents.

[seanh]

Alright, I made a few small suggestions but I think let's go ahead and get this merged.

For future PRs I think there'd be value in adding direct unittests for the rest of the helper functions in core.py: looks like currently only notify() and _get_search_after() have unittests.

Also think there'd be value in adding docstrings to the various functions.