Fetch FundingInstrument and Return the appropriate resource

This saves people from metaprogramming module names `::Card` or `::BankAccount` based on hrefs.
You mean so you don't have to add something like:

````ruby
  Balanced::Bitcoin.fetch(href) if href.include?('bitcoin')
````

Or whatever the new payment may be? 

I would tend to agree with you on building something that allows for more FundingInstruments to be added elsewhere as resources and this module just adapts. Rather than having to explicitly call out each payment method in an `if` statement.

The only pushback I'd give is... what's the likelihood of adding new funding instruments in the future? And should this code be optimized for that?