Optional colorized output, ability to squelch log prefixes, and support for graceful terminations of child process

I added a -color command line flag to enable coloring of daemon output success/failure messages. Currently this adds another dependency on github.com/fatih/color . If you'd rather not have another dependency, I'd be happy to pull out just the parts of that library that are used in this pull request.

I added a -log-prefix flag to control whether or not the output from the daemon includes log timestamps and the "stderr:"/"stdout:" labels. When combined with colorized output from the daemon process, I find that having additional logging info just pollutes the output.

I added a -graceful-kill flag that adds support for sending the child process a SIGTERM before outright killing it. This will give the child process a chance to correctly clean up any resources it may have allocated.

All of these new options are set to defaults that keep the behavior of CompileDaemon functioning as it did before these changes.
The `_` at the beginning of the line is not really a standard notation. Can you rename `_okColor` and `_failColor` to their respective names without the `_`? A simple `gofmt -r '_okColor->okColor' daemon.go` should suffice.