Allow a JSON input parameter for the `generate` command

[aaroncox]

Currently you can use the generate command with the --url flag to pull the ABI from chain.

wharfkit generate --url=https://jungle4.greymass.com eosio.token

And it'll pull the ABI from chain and generate the code.

As an alternative syntax, we should allow JSON to be directly fed to the script to perform the same function.

wharfkit generate --json=abi.json eosio.token