Porting xstream-boilerplate to create-cycle-app flavor

[nickbalestra]

As per discussion on cyclejs-community/xstream-boilerplate#233

[geovanisouza92]

I was thinking, as xstream-boilerplate is, as name suggests, xstream-focussed, this could raise the idea about limiting flavors by its supported stream-libs, otherwise, this will force flavors to support multiple stream libraries.

For the core flavors, this is a must, but for private/personal flavors that could be used by anyone, this could be annoying, considering that, if I prefer to use one library for my projects, this would be the preferred one for all projects.

[Widdershin]

I would also quite like to limit my flavor to xstream only.

[nickbalestra]

Yes, I totally agree. We should have a way to opt-out from streamlib selection, in the case that a flavor only want to support one (it makes a lot of sense indeed for custom flavors), added an ad-hoc issue for that: #33