Add hot/cold signal feature.

This pull request will add "hot/cold signal" functionality using SwiftTask's [paused-init feature](https://github.com/ReactKit/SwiftTask/pull/17).

All signals are now preferred "cold signal :snowflake:"  by default, 
and will "let it go" (auto-activate) on `<~` (progress), `then()`, `success()`, and `failure()` chaining.

Related: https://github.com/ReactKit/SwiftTask/pull/17