Implement Williams %R Strategy

[cinar]

Implement the Williams %R trading strategy using the existing momentum.WilliamsR indicator.

Location: strategy/momentum/williams_r_strategy.go

Strategy Rules:

• Buy Action: Williams %R crosses above -80 (recovering from oversold).
• Sell Action: Williams %R crosses below -20 (recovering from overbought).
• Hold Action: Otherwise.

Implementation Details:

• Follow the strategy.Strategy interface.
• Use momentum.NewWilliamsR() or allow custom parameters.
• Provide unit tests with historical data in strategy/momentum/testdata/williams_r_strategy.csv.
• Ensure 100% test coverage.