Make web demo optional

[TilCreator]

I'm currently trying to package this project for NixOS, mostly for use with speech-dispatcher.

The web demo is a nice feature, especially for testing with the docker container, but it seams like bloat when only using the command line.

It would be nice to have the web demo as an optional module or as a differed module with Larynx as a dependency.

It's a really awesome project and I hope to use it soon!