benchmark: add filter option for benchmark

Before:

    # common.js executes all tests in net directory.
    $ ./iojs common.js net

After:

    # common.js executes only "dgram" tests in net directory.
    $ ./iojs common.js net dgram

PR-URL: https://github.com/iojs/io.js/pull/488


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>