Allow method chaining in @sendgrid/mail

[reichert621]

Issue Summary

Very minor, but it would be nice to allow method chaining in the MailService of @sendgrid/mail so that setting the API key could be handled on one line:

const mailer = require('@sendgrid/mail').setApiKey(process.env.SENDGRID_API_KEY);

// vs

const mailer = require('@sendgrid/mail');

mailer.setApiKey(process.env.SENDGRID_API_KEY);

This is all it would require as far as I can tell:

class MailService {
  // ...

  setClient(client) {
    this.client = client;

    return this; // TODO (optional, but good for consistency)
  }

  setApiKey(apiKey) {
    this.client.setApiKey(apiKey);

    return this; // TODO
  }

  setSubstitutionWrappers(left, right) {
    // ...

    return this; // TODO (optional, but good for consistency)
  }

  // ...
}

(It looks like this pattern is used here in @sendgrid/client https://github.com/sendgrid/sendgrid-nodejs/blob/master/packages/client/src/classes/client.js#L53-L67 as well.)

Steps to Reproduce

1. Install @sendgrid/mail
2. Set the API key

Technical details:

• sendgrid-nodejs version ^6.2.1

[thinkingserious]

Thanks for the suggestion @reichert621!

I've added this to our backlog for consideration.

Please feel free to submit a PR as well :)

[AllUsernamesHaveBeenTaken]

Hello, can I handle this one? Thank you!

[thinkingserious]

Thank you @AllUsernamesHaveBeenTaken! It's all yours.