Add option to export result to json file instead of stdout

[maxidev]

For example

$ btcxp addr 17WRjamox6VhTUaHsTWfFnMNDYHvwCtWio --tojson

creates out.json
{

Address: "17WRjamox6VhTUaHsTWfFnMNDYHvwCtWio"
Address Type: "Legacy (P2PKH)"
ASM: "OP_DUP OP_HASH160 476176f30aa8daa62554af8da75d801f68a58f42 OP_EQUALVERIFY OP_CHECKSIG"
Balance: "0.5069768 BTC (~ 16236.54 USD)"

}