SS58 versioning (Network IDs) (#3147)

* Introduce network IDs for SS58

* Fix

* Allow numeric overrides.

* Improve docs

* String rather than str

* Comment out code that will become valid after other PR

* Fix

substrate/subkey/src/cli.yml

@@ -18,6 +18,12 @@ args:
       takes_value: true
       required: false
       help: The password for the key
+  - network:
+      short: n
+      long: network
+      takes_value: true
+      required: false
+      help: Specify a network. One of substrate (default), polkadot and kusama.
 subcommands:
   - generate:
       about: Generate a random account