Allow the user to specify the host/port for the servers explicitly.
Allow the user to specify the host/port for the servers explicitly.

Rather than just having dev/prod options (though those are allowed too).

Auditors: jlfwong

Test Plan:
Both of these ran without a syntax error:
   tools/benchmark_ka_component.js --dev-render
   tools/benchmark_ka_component.js --render-host http://localhost:8431

Khan/react-render-server,Khan/react-render-server,Khan/react-render-server