Calculate and expose list of parameters for the combination of plugins

For a long time now, it is also possible to chain several plugins.

The /plugins API returns the parameters for each plugin, but there is no easy way to know what parameters can be sent to the chain. There may be some discrepancies between the values that each plugin accepts.

I propose the following:

• Accept the union of both sets of parameters
• For shared parameters, provide the intersection of valid options
• Allow setting different parameters for each plugin via special names. e.g. for plugin sentiment, one could set its language parameter with sentiment-language.

The parameters should be listed with the API as well. #50 (closed) should probably be implemented first.

Note that parameter validation/handling on plugins execution should also change, but I think it will simplify things for the client and the UI.