Make module express their features

[sthibaul]

Is your feature request related to a problem? Please describe.
The server code has hardcoded lists such as

		int punct_missing = 0;
		if (strcmp(output->name, "flite") == 0 ||
		    strcmp(output->name, "dtk-generic") == 0 ||
		    strcmp(output->name, "epos-generic") == 0 ||
		    strcmp(output->name, "llia_phon-generic") == 0 ||
		    strcmp(output->name, "mary-generic") == 0 ||
		    strcmp(output->name, "swift-generic") == 0 ||
		    strcmp(output->name, "pico") == 0)
			/* These don't support punctuation */
			/* FIXME: rather make them express it */
			punct_missing = 1;

Describe the solution you'd like
The modules should expose their features to the server. We'd want that after initialization. This could be a new command that the server sends to the module to get a feature list, which could be a mere list of lines. The features that the server doesn't understand can be ignored, and if the client says it doesn't support the command, we can assume that it doesn't support the features.

That would probably be useful for exposing the punctuation support as mentioned above, but also ssml support, see #905