add support for categories

[elesdoar]

This commit provides support for categories with option categories.

var email = {
    to: ['[email protected]', '[email protected]'],
    from: '[email protected]',
    subject: 'Hi there',
    text: 'Awesome sauce',
    html: '<b>Awesome sauce</b>',
    categories: 'category-a,category-b'
};

or

var email = {
    to: ['[email protected]', '[email protected]'],
    from: '[email protected]',
    subject: 'Hi there',
    text: 'Awesome sauce',
    html: '<b>Awesome sauce</b>',
    categories: ['category-a', 'category-b']
};

[thinkingserious]

Awesome! We've added this to our backlog for review.

[RobAWilkinson]

would love to see this merged!

[thinkingserious]

Hello @RobAWilkinson,

We no longer support this library. The official SendGrid Node.js client library can be found here: https://github.com/sendgrid/sendgrid-nodejs

Thanks!

[glesage]

FYI just rebuilt this module for the new Sendgrid 4.x framework, and supports categories...

It definitely lacks more thorough tests but it'll do (: #28