Mimic cargo adding -p flag to build sub contracts, and all with --all

[ascjones]

Further to #174 and in relation to #60, cargo-contract should mimic cargo for building/testing individual contract packages.

At the moment you can build a sub contract workspace using --manifest-path=subcontract/Cargo.toml, but it should also be possible to do -p subcontract.

Similarly we should be able to build/test all contracts in the workspace with --all