More succinct configuration of DoubleEntry

What was:
```ruby
DoubleEntry.accounts = DoubleEntry::Account::Set.new.tap do |accounts|
  accounts << DoubleEntry::Account.new(identifier: :savings,  positive_only: true)
  accounts << DoubleEntry::Account.new(identifier: :checking)
end

DoubleEntry.transfers = DoubleEntry::Transfer::Set.new.tap do |transfers|
  transfers << DoubleEntry::Transfer.new(from: :checking, to: :savings,  code: :deposit)
  transfers << DoubleEntry::Transfer.new(from: :savings,  to: :checking, code: :withdraw)
end
```
Is now:
```ruby
DoubleEntry.configure do |config|
  config.define_accounts do |accounts|
    accounts.define(identifier: :savings,  positive_only: true)
    accounts.define(identifier: :checking)
  end

  config.define_transfers do |transfers|
    transfers.define(from: :checking, to: :savings,  code: :deposit)
    transfers.define(from: :savings,  to: :checking, code: :withdraw)
  end
end