Add support for batch record creation

[EvanHahn]

Note: batch record operations are currently beta. Please don't use this for production workflows yet.

This PR adds support for batch record creation. For example:

const table = airtable.base('app123').table('My Table');
const records = await table.create([
    {foo: 'boo'},
    {bar: 'yar'},
]);

console.log(records.length);        // => 2
console.log(records[0].get('foo')); // => 'boo'
console.log(records[1].get('bar')); // => 'yar'

The typecast option works the same as before:

await table.create([/* ... */], {typecast: true});