Demo five: Topics

With direct exchange can't do routing based on multiple criteria.
We may want to listen to just critical errors coming from 'cron' but also all logs from 'kern'. To implement that in our logging system we need to use topic exchange.
The logic behind the topic exchange is similar to a direct, However there are two important special cases for binding keys:
- * (star) can substitute for exactly one word.
- # (hash) can substitute for zero or more words.

Running Locally

To receive all the logs:
```
node consumer.js "#"
```
To receive all logs from the facility 'kern':
```
node consumer.js "kern.*"
```
Or if you want to hear only about 'critical' logs:
```
node consumer.js "*.critical"
```
You can create multiple bindings:
```
node consumer.js "kern.*" "*.critical"
```

And to emit a log with a routing key 'kern.critical' type:

node producer.js "kern.critical" "A critical kernel error"