Improve OAuth1 support

[bookmoons]

Raises a descriptive error if insufficient OAuth1 configuration is available. I'm not finding a clear description of which config is required. Went for the set that's nearest I could tell:

To convert this collection provide OAuth credentials. Either include them in the collection or use the --oauth1-* CLI options. Minimum required configuration is signature method, consumer key, consumer secret, access token, and token secret.

Adds command line options for specifying OAuth1 configuration. CLI options override settings in the collection.

  --oauth1-consumer-key <value>      OAuth1 consumer key.
  --oauth1-consumer-secret <value>   OAuth1 consumer secret.
  --oauth1-access-token <value>      OAuth1 access token.
  --oauth1-token-secret <value>      OAuth1 token secret.
  --oauth1-signature-method <value>  OAuth1 signature method. One of HMAC-SHA1 HMAC-SHA256 PLAINTEXT.
  --oauth1-timestamp <value>         OAuth1 timestamp.
  --oauth1-nonce <value>             OAuth1 nonce.
  --oauth1-version <value>           OAuth1 version.
  --oauth1-realm <value>             OAuth1 realm.

This command converts the twitter.json example:

node bin/postman-to-k6.js example/v2/twitter.json --oauth1-signature-method HMAC-SHA256 --oauth1-consumer-key ConsumerKey --oauth1-consumer-secret ConsumerSecret --oauth1-access-token AccessToken --oauth1-token-secret TokenSecret

Closes #20.

[robingustafsson]

LGTM, merging. Thanks @bookmoons!