Implement a more automated method of declaring signals

[treethought]

While it is currently possible to provide custom signal logic using the Strategy decorators, the ability to declare signals inside JSON would likely be more useful for machine learning than declaring them inside python scripts.

[treethought]

It may be pretty difficult to allow defining custom signals in JSON, as some may be complex or not easily translated. But I think we can provide a set of common signals and potentially combine them. We can start by improving the signals utils module with more complicated/useful signals, and reference those signal functions in the JSON.

@Splentich, do you have any knowledge of potential signals we could implement?

[slavakurilyak]

Creating a common set of signals is a great idea! Here are four additional signals to consider: Rising / Falling, N Bars Up / N Bars Down (Inspiration: NinjaTrader, 2018).