lib: add setFlagsFromString() to tracing module

Expose v8::V8::SetFlagsFromString() on tracing.v8 in lib/tracing.js.

PR-URL: https://github.com/node-forward/node/pull/62


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>