Allow both .abi files an truffle artifacts when using load-contract

[fvictorio]

The load-contract command requires as first argument a path to the ABI file of the contract.

It would be nice to allow using truffle artifacts files too. It shouldn't be hard: if the given file has an array, interpret it as an ABI file. If it's an object and it has an abi propery, interpret it as a truffle artifact.

[fvictorio]

Closed by #10.