Add option to automatically create a topic and re-use a subscription.

[theacodes]

This adds autoCreate to pubsub.topic:

var topic = pubsub.topic('does-not-yet-exist', {autoCreate: true});
// topic will be created on first RPC.
topic.publish({data: {hello: 'world'});

And reuseExisting to topic.subscribe:

topic.subscribe('existingSub', {reuseExisting: true}, function(err, sub){ ... }));

Combined, these two features completely eliminate any bootstrapping code/steps required for the Getting Started on GCP sample.

[theacodes]

Fixed the lint issues and updated after the review.
Also, forgot to mention in the PR - this fixes #445.

[ryanseys]

Overall LGTM. One stylistic nitpick: we add spaces between our keys in an object.

Like this: { hello: 'world' } not like this: {hello: 'world'}

[stephenplusplus]

LGTM. Thanks for doing this!

[ryanseys]

Merged in 3f8f60f

[matthewmueller]

I wasn't able to get this working from the example you provided. Here's my code:

const PubSub = require('@google-cloud/pubsub')

const pubsub = PubSub({
  projectId: require('./cred.json').project_id,
  credentials: require('./creds.json')
})

const topic = pubsub.topic('hi-there', { autoCreate: true })

topic.publish({
  data: 'New message!'
}, function(err) {
  console.log(err)
});

pubsub.topic doesn't actually take an 2nd options parameter

[stephenplusplus]

A similar example came up on StackOverflow (http://stackoverflow.com/questions/38864043/gcloud-check-if-a-topic-exist-and-ability-to-reuse-the-topic), so I'll quote my response there:

I believe the problem you'll run into is that if a message is published to a topic that doesn't exist, it is immediately dropped. So, it won't hang around and wait for a subscription to be created; it'll just disappear.

However, gcloud-node does have methods that will create a topic if necessary:

var topic = pubsub.topic('topic-that-maybe-exists');
topic.get({ autoCreate: true }, function(err, topic) {
  // topic.publish(...
});

In fact, almost all gcloud-node objects have the get method that will work the same way as above, i.e. a Pub/Sub subscription or a Storage bucket or a BigQuery dataset, etc.

Here's a link to the topic.get() method in the docs: https://googlecloudplatform.github.io/gcloud-node/#/docs/v0.37.0/pubsub/topic?method=get

[gustawdaniel-acaisoft]

I have error

TS2345: Argument of type '{ autoCreate: boolean; }' is not assignable to parameter of type 'PublishOptions'.   Object literal may only specify known properties, and 'autoCreate' does not exist in type 'PublishOptions'.

[gustawdaniel-acaisoft]

these are publish options

export interface PublishOptions {
    batching?: BatchPublishOptions;
    flowControlOptions?: FlowControlOptions;
    gaxOpts?: CallOptions;
    messageOrdering?: boolean;
    enableOpenTelemetryTracing?: boolean;
}