[GH-121] Add Aroon indicator

[jealous]

Add aroon oscillator.

• Aroon Oscillator = Aroon Up - Aroon Down
• Aroon Up = 100 * (n - periods since n-period high) / n
• Aroon Down = 100 * (n - periods since n-period low) / n
• n = window size

https://www.investopedia.com/terms/a/aroonoscillator.asp

The default window is 25.