lib: reintroduce v8 module

I introduced this module over a year ago in a pull request as the v8
module but it was quickly subsumed by the tracing module.

The tracing module was recently removed again and that is why this
commit introduces the v8 module again, including the new features it
picked up commits d23ac0e and f8076c4.

R=@trevnorris
I'd be in favor of something like:

```markdown
The V8 options available for a version of node may be determined by running `node --v8-options`. An unofficial, community-maintained list of options and their effects is available [here](https://github.com/thlorenz/v8-flags/blob/master/flags-0.11.md).
```

Since it stresses that the linked guide isn't the "source of truth" -- that lies within v8 -- but gives casual readers a chance to see what's available through this API.

(Also, great work @thlorenz!)