Allow clients to pass arbitrary configuration parameters

**Is your feature request related to a problem? Please describe.**

from @albertotirla :

“the espeak-ng module doesn't even support proper inflection, same for rate boost”

“spd doesn't allow the client application to pass arbitrary parameters to the synth and the synth to report an error if those make no sense or just a warning and ignoring them”

**Describe the solution you'd like**

It's not exactly clear how we want to fix this.

- For some parameters which also make sense to other modules, we can introduce them more generally, and they can appear in the Orca configuration panel
- For other parameters which are really specific to espeak-ng, we may not want to clutter the Orca configuration panel with yet more knobs. Is it fine to just add support for them into `espeak-ng.conf`?