structured usage rates

Add structured usage rates for multiple currency and personalized messages.

For example, here is a manifest.yml fragment:

    Usage-Rates:
      Scope: gear
      Duration: month
      Rate:
        eur: 0.03
        usd: 0.05
        brl: 0.10
      Description:
        en-us: "This gear costs an additional %{currency}%{rate} per gear after the first 3 gears."
        pt-br: "Este gear custa %{currency}%{rate} adicionais por gear após os primeiros 3 gears."

Valid variables to `Description` are:
- currency: currency symbol used ($, R$, €...)
- rate: used rate
- scope: applied scope for this rate (ex: account, domain, application, gear)
- duration: timespan this charge applies to (ex: hour, month, once)