adding structured output option

[tomreitz]

This PR implements the structured output portion of this issue. (A separate one will be coming to implement the exit code portion.) It adds the --results-file flag which allows you to specify an output to file to which earthmover will write a structured JSON run summary.

Example JSON output can be found in the addition to the README in this PR.

[jayckaiser]

Consider isolating the failed statuses dict as a variable to streamline updates.

failed_statuses_dict = self.metadata["resources"][endpoint].get("failed_statuses", {})

if response.status not in failed_statuses_dict:
    failed_statuses_dict.update({response.status: {}})

if message not in failed_statuses_dict[response.status]:
    failed_statuses_dict[response.status].update({message: {}})

if "files" not in failed_statuses_dict[response.status][message]:
    failed_status_dict[response.status][message].update({"files": {}})

if file_name not in failed_statuses_dict[response.status][message]["files"]:
    failed_statuses_dict[response.status][message]["files"].update({file_name: {}})

if "line_numbers" not in failed_statuses_dict[response.status][message]["files"][file_name]:
    failed_statuses_dict[response.status][message]["files"][file_name].update({"line_numbers": []})

failed_statuses_dict[response.status][message]["files"][file_name]["line_numbers"].append(line)

Additionally, this is a very example of a defaultdict use-case, but it's multiple layers nested which might be difficult to set-up. If we were able to do so, it would convert this entire code-block to the following:

self.metadata["resources"][endpoint]["failed_statuses"][response.status][message]["files"][file_name]["line_numbers"].append(line)

I'm going to play around with this concept a little more.

[tomreitz]

I see what you're saying, and this could be cleaner. One little bug in what you posted above, I think I still need to check for values in objects' .keys(), since they're nested dicts. You have

if response.status not in failed_statuses_dict:
    failed_statuses_dict.update({response.status: {}})

which I think still needs to be

if response.status not in failed_statuses_dict.keys():
    failed_statuses_dict.update({response.status: {}})

(etc.)

[tomreitz]

@jayckaiser I made another update along these lines in this commit and I tested it. If this is good enough for now, I can merge and we could consider refining later?