title: Set unit for number_to_currency with a user setting?
body: <p>I want to allow users to change the currency unit throughout their account.</p>
  
  <p>The obvious way to do it is to pass the <code>unit</code> parameter to <code>number_to_currency</code>, but given <code>number_to_currency</code> is used hundreds of times throughout the app, it seems a little repetitive to do that.</p>
  
  <p>So is there some way to change what unit is used for all instances of <code>number_to_currency</code> based on a setting stored in the database for each user?</p>