improve "build on run" UX for source commands

[progrium]

We support "build on run" for commands implemented as Go source directories, but we get the output of the build compiler when its run. The build compiler should probably only output when there is a problem and leave the progress output behind a verbose flag. This would make "build on run" a more seamless experience.

[Parzival-3141]

As of 214113a this is "technically" complete, though only because all build output gets ignored in proc.go. AFAICT this is blocked by #64 since we don't have a good way of using stdio from within the kernel. In the meantime we could workaround this by piping build's stdout/err to the browser console.

[progrium]

I recently set the default stdio for the kernel to be logged with console.log so this is good for now.