Return version from package.json on CLI, fixes #415 and bitpay/bitcore#1368

[kingjan1999]

One might argue that reading version numbers from package.json isn't good style and may have security implications (https://stackoverflow.com/a/10855054), but I don't see any problems here.

This fixes #415 and should fix bitpay/bitcore#1368 (though I haven't tested the latter).