Small bugfixes & features

[cspotcode]

Splitting #40 into multiple PRs for easier review.

[AndreasMadsen]

I don't know what this fixes. To me, it adds two unrelated features.

I'm okay with the "async boundary" line, it has been requested a few times, but it has to be implemented properly to ensure compatibility with other modules. See for example AndreasMadsen/stack-chain#7

I'm closing this for now. If you feel like implementing AndreasMadsen/stack-chain#7, then we can reopen this.

[cspotcode]

The two other changes are also from #40:

• adding a disable() method to stop tracing, opposite of enable()
• Fixing the logic that excludes async_hooks.js stack frames.

For the latter, it looks like this library intends to remove all "async_hooks.js" stack frames from traces. In node v11, there's an "internal/async_hooks.js", so if you want to filter out those frames, an additional equality check is required.

They're both tiny, so if anyone wants to get them merged, it should be easy to cherry-pick into separate PRs.