Chainable set

This release modifies set to returned the instance, so that you can chain set calls. This is useful for constructing and setting the initial values of a Topologica instance in a single statement.

Before:

const state = Topologica({ circlesDOM });
state.set({ selection });
return state;

After:

return Topologica({ circlesDOM }).set({ selection });

PR: Chainable set #35