src: override v8 thread defaults using cli options

Based on the conversation in #4243 this implements a way to increase
and decrease the size of the thread pool used in v8.

Currently v8 restricts the thread pool size to `kMaxThreadPoolSize`
which at this commit is (4). So it is only possible to
decrease the thread pool size at the time of this commit. However with
changes upstream this could change at a later date.
If set to 0 then v8 would choose an appropriate size of the thread pool
based on the number of online processors.

PR-URL: https://github.com/nodejs/node/pull/4344


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>