new subcommand uses `contract.dhall` as default directory path

[shonfeder]

defaultContractPath, as defined at https://github.com/informalsystems/themis-contract/blob/master/cmd/themis-contract/root.go#L14, is contract.dhall. When no output argument is given to the new subcommand, it defaults to the defaultContractPath, and then tries to create a directory with this name, but we don't want our directory to be called contract.dhall.

Perhaps this should default to .?