Added -j/--json command to output data as JSON object

I thought it would be useful to have the option to output the genwallet data as a JSON object, for easy import into other scripts/projects of various languages.

e.g. $ genwallet -j -u
{
  "wallet_key" : "xprv9s21ZrQH143K3vdSSNg73eFNPcKiZovpr7M5VpawvLFJcxBJGWSMfEEBQrW7b2n5vK7UgkMKhqHKjHXHvqd2tDQskEat4keSu4mL9rfZJ9x",
  "network" : "main",
  "key" : "private",
  "secret_exponent" : "52173516184573901704032841137943751493621219238054698718425027791275381833118",
  "public_pair_x": "7072653023074047502993803083583129538202139581355607368574533469656301109201",
  "public_pair_y" : "68914524858460888550750890196693321650849613189039390288972841961149959319586",
  "tree_depth" : "0",
  "fingerprint" : "3997545e",
  "parent_fingerprint" : "00000000",
  "child index": "0",
  "chain code": "bb8004d4deeedc534f882eeca7c974ebc15596b9e20ce6ee635b13eb5c8c02f4",
  "WIF" : "L15w4Wf8XDktqxESTHfq1KGJGfLMMot69fjVYC9BdUQsqy9FwQwt",
  "WIF_uncompressed" : "5Jh5ztaZWxTFvERLpC2Yim1kLQe1XzkJ2Wcugh8kByxhyUfzdwg",
  "bitcoin_addr" : "16FWoVop69nRQuMQsdYcXXxPbwAiyKp9up",
  "bitcoin_addr_uncompressed": "18YMacT6hk1f4ZGYw6s5u7p3GBd6Jjs9gQ"
}