benchmark: fix configuation parameters

The benchmark runner spawns new processes for each configuration. The
specific configuration is transfered by process.argv. This means that
the values have to be parsed. As of right now only numbers and strings
are parsed correctly. However other values such as objects where used.

This fixes the benchmarks that used non-string/number values and
prevents future issues by asserting the type.

PR-URL: https://github.com/nodejs/node/pull/5177


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>