[Bug] Add support for CLI to handle rule ndjson files with extra metadata

[brokensound77]

Describe the bug
Exported rules which are in ndjson format have extra metadata which causes importing to fail for several CLI commands.

• create-rule -c
• load-from-file
• view-rule

{"actions":[],"created_at":"2020-07-21T13:52:18.527Z","updated_at":"2020-07-21T13:52:18.544Z","created_by":"elastic","description":"This is a demo of an example rule","enabled":false,"false_positives":["Sometimes cmd is benign :)"],"filters":[],"from":"now-360s","id":"90f6cdcb-6211-4985-ab75-c6253fcdd868","immutable":false,"index":["winlogbeat-*"],"interval":"5m","rule_id":"6b9d2af4-84c8-4e38-a697-2d5d455a9e86","language":"kuery","output_index":".siem-signals-default","max_signals":100,"risk_score":50,"name":"Example Custom Rule","query":"process.name:cmd.exe and process.args:\"-c\"","references":["https://google.com"],"meta":{"from":"1m","kibana_siem_app_url":"https://703efb61c6ee4b14b36f7f4a9674a5ea.us-west-2.aws.found.io:9243/app/siem"},"severity":"low","updated_by":"elastic","tags":["Windows"],"to":"now","type":"query","threat":[{"framework":"MITRE ATT&CK","tactic":{"id":"TA0002","reference":"https://attack.mitre.org/tactics/TA0002","name":"Execution"},"technique":[{"id":"T1064","name":"Scripting","reference":"https://attack.mitre.org/techniques/T1064"}]}],"throttle":"no_actions","note":"## Triage\nWhy are they executing `cmd.exe`?","version":1}
{"exported_count":1,"missing_rules":[],"missing_rules_count":0}

To Reproduce
Steps to reproduce the behavior:

1. export a rule from kibana
2. python -m detection_rules -c export.ndjson new_rule.toml --required-only

Expected behavior
The CLI should be updated to be able to understand importing the ndjson formatted rules and ignore extra data. It currently has the ability to parse json, toml, and yaml.

Rules will still need to be able to validate against the schema to be accepted

Additional context
related to #76