Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion; sauté 3 minutes. Add cumin, chili powder, shrimp, and garlic; sauté 4 minutes. Remove from heat, and add butter, salt, and hot sauce. Stir until the butter melts. Stir in juice and green onions. Garnish with the lime wedges, if desired.