Make testing extractors easier

[diptanu]

New Extractors can be tested by running the server and by binding them to a repository. This is cumbersome because the feedback cycle is not fast.

Solution -
Create a sub-command under the indexify binary to run just an extractor

indexify executor run-extractor --python-module foobar.ExtractorClassName --content text-from-which-we-are-extracting --params `{"foo": "bar"}`

The above example allows a developer who is developing a new extractor in footer.ExtractorClassName to load the extractor on their terminal pass some text to it and also the input parameters of the extractors encoded in json.

In the future when we add support for images, videos, or any other blobs, we could enhance this to pass the pointer to files.