unimplemented agg: terms - significant_terms

[jameslamb]

Currently, this type of aggregation is not supported by the Python package. "not supported" means that es_search() cannot parse a result from this type of query into a pandas DataFrame.

It's possible that this is handled easily and correctly by pandas.DataFrame.from_json().

To close this issue, a PR would need to:

1. implement unit tests similar to the corresponding R tests in r-pkg/tests/testthat/test-chomp_aggs.R. See the test results in test_data/. There is one file there corresponding to each aggregation type (or combination of agg types) mentioned in README.md
2. update the corresponding line in the agg support table in README.md
3. if necessary, add code to to fetch_all.py. Parsing functions should be implemented as internal functions (with leading underscores) to ensure that es_search() stays magical. Reference the R implementation to see how this could be done.