Log command before running it

When Collector runs a command, it passes the output from that command directly to the terminal from which Antora was launched. As a result, the user will see output from commands, but not know where the output is coming from.

To help the user understand and debug the behavior of commands, Collector should report the command it is going to run. This will act as a header for the output that follows. It should include the same context information that is provided when a command fails (url, refname, start path, etc).

Since the command is permitted to write to stdout, it seems reasonable to output the command itself to stdout as sort of a header. We may also consider using Antora's logger to log the command to the info level.