Allow custom currencies

Kukks

Please allow us to define custom currencies. This would help with issues revolving around payments made in Bitcoin and other obscure currencies or digital platform token. With custom currencies, you can create accounts denominated in that currency. Since you do not have any rate provider source and to keep the scope simple enough, I propose that any multi-currency transfer recorded requires the user to input the exchange rate explicitly.

Issues this could help solve:
https://community.waveapps.com/discussion/681/support-for-cryptocurrencies
https://community.waveapps.com/discusion/4853/cryptocurrencies

I currently have to start evaluating other platforms to support this and it is going to be a massive pain to migrate.