Change the information that is displayed when shelling out.

In `warn` mode, Omnibus will only display warnings/deprecations; you will not
see any build commands or output. In `info` mode, Omnibus will display the
commands and environment that are being used; you will not see the output from
the build (unless it fails). In `debug` mode, Omnibus will display the
commands, environment, and output (livestream) from commands.