fix: add start method

[JGAntunes]

The lack of start implies that events might get lost since we have no way of setting up handlers before ping starts emitting.

[JGAntunes]

@diasdavid if you could review this when you have some please ☝️

[daviddias]

No tests? What was failing?

[JGAntunes]

@diasdavid what happens is that the ping may start emitting events before handlers can be setup. Specifically - https://github.com/JGAntunes/js-ipfs/blob/feat/ping/src/core/components/ping.js#L65 - this error may not be caught since this - https://github.com/libp2p/js-libp2p-ping/blob/master/src/ping.js#L28 - might execute before the handlers are registered.
I can try to add tests to this but it's just that it looks like a kind of a complicate race condition that comes from the way the module is structured.

[daviddias]

@JGAntunes the event emitter is created before .start is called, you can always set up the handler there. Am I missing something? Anyhow, merging and releasing this one so that it can unlock the ping PR

[JGAntunes]

@diasdavid sorry, I'm not following, before you had no start, which meant that the swarm.dial would be called on ping construction and stuff like this - https://github.com/libp2p/js-libp2p-ping/blob/v0.7.0/src/ping.js#L28 - for example could be emitted before you had time to setup event handlers.