Add parameters to the playground

Things like language, outformat and the like should be in the playground.

Instead of adding them manually, we should export API_PARAMS, WEB_PARAMS and NIF_PARAMS and generate the form parameters automatically.

Subtasks:

Add 'help' as a parameter to the API, with two options: "true" and "false". It is false by default.
Modify all the /api blueprint to export the list of available params (API_PARAMS, WEB_PARAMS, etc.) as a JSON file if given "help=true". (something like `/api/params/
Use the exported values to generate the form in the playground
Group plugins by pluginType in the dropdown menu (with an optgroup)

Edited Jul 17, 2017 by Manuel García-Amado