CIP-0030 | Upgrade to latest format & propose version / extension scheme (cardano-foundation#446)

* Upgrade CIP-0030 to latest format & propose version / extension scheme

  This has been a recurring topic regarding extensions of CIP-0030.
  Given that this CIP is effectively active (hence the status change in
  this commit), we can't simply keep modifying it because it then
  becomes hard for DApps to keep track of what is supported by wallet
  providers and what isn't.

  In this commit, I henceforth propose an extension scheme based off
  CIPs. Wallet providers are expected to support the base API given by
  CIP-0030 and new extensions or modifications of that API can be
  brought in through extension CIPs. Upon initialization, Dapps can
  interrogate the wallet about what CIPs extensions it supports and
  process or fail accordingly based on whether the wallet provides the
  desired functionality.

* Make 'enable' parameter an object to favor extensibility.

* Make CIP-0030 as 'Active' in top-level README.

Author: KtorZ