Add support for Monero (XMR) stagenet addresses

[septagram]

Aside from prod and testnet, Monero also has stagenet which is essentially the same thing most other cryptocurrencies call "testnet". In Monero, however, testnet is the "experimental" network and blockchain where things get released long before mainnet.

[septagram]

It says "unknown repository" because I deleted my fork so I can create a similar pull request at christsim/multicoin-address-validator, which appears to be better maintained at the moment :(

[aaronovz1]

This also needs to include validation of subaddresses. See https://github.com/monero-project/monero/blob/master/src/cryptonote_config.h for the prefixes.

[septagram]

Thanks for the pointer, will try to implement that, it's definitely necessary.

However, the original code did did not include subaddress validation as well, so IMHO subaddress validation can be added in a separate pull request.