Enable ink-debug feature for enabling debug messages

[ascjones]

In use-ink/ink#792 we introduce the ink-debug feature, which needs to be enabled in order for debug_print! messages to be sent to the supervisor and printed to the console/sent back to the calling client.

So we should allow the enabling/disabling of this feature by introducing the --debug and --release builds or some other way to control it. It may not assume that the contract has that feature added to its Cargo.toml, so could add as necessary during the build.

Perhaps in addition we should allow manual control of features passed to the underlying build command in case the user wants full control.