integrate subkey-like commands

subkey is the substrate key utility. It is convenient to have and should be distributed with gcli. Example

subkey generate
Secret phrase:       whisper stereo language wrestle romance guilt bomb devote glimpse teach senior forget
  Network ID:        substrate
  Secret seed:       0xeb1450862f6a18350726bb6ddd2bbcd43442b2bccc3bff5e73bef02b3df44aaa
  Public key (hex):  0x9e6e4e8283acc8262323bc20286760df95e5e4bd927372b8ce59ada0c303574c
  Account ID:        0x9e6e4e8283acc8262323bc20286760df95e5e4bd927372b8ce59ada0c303574c
  Public key (SS58): 5FeSC2M3hEFnkM4v8rzXVNa9P3hxipAYik4TJSn58jse65NA
  SS58 Address:      5FeSC2M3hEFnkM4v8rzXVNa9P3hxipAYik4TJSn58jse65NA