Fix bad calculations; add syntactic sugar

• Improve MRR calculations with prorated churned and new MRR (hopefully fixes bad churned MRR calculations)
• Only consider paid charges for all revenue calculations (hopefully fixes bad ARPC calculations)
• Add at: parameter as a named parameter option for estimated_valuation, like (estimated_valuation(at: "3x")
• Add multiple: parameter as another option for estimated_valuation (same as at:, just syntactic sugar)
• Now estimated_valuation also supports Numeric-only inputs like estimated_valuation(3)

🎉 Initial release!

Provides essential methods to calculate SaaS metrics like MRR, ARR, LTV, ARPC, estimated valuation, churn, etc. – with a simple dashboard to display them.