support "processes" to handle extra procs

[dbushong]

Now you can add something like this to your .testiumrc:

{
  "processes": {
    "memcached": {
      "command": "memcached",
      "commandArgs": ["-u", "memcached", "-d"],
      "port": 11211,
      "reuseExisting": true
    }
  }
}

...and that will add a new server to the set to be started before tests run.
The properties inside the processes option are the server names, and all of the other options
are those for subprocess, except for reuseExisting.

Iff reuseExisting is set to true, and something is already listening on the given (now
mandatory) port, then the command and commandArgs will not be run.

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This PR was started by: git wf pr

[erothman-groupon]

👍