Set default log to src-cli.debug.log if build tag debug is used

Created by: mrnugget

I don't care about the specifics - filename, build tag, etc. - but I need this. And the reason why I need it is that it's hard to log something when our TUI is active. Not because of how the TUI is implemented, but because the whole concept of an updating TUI doesn't gel with printf-debugging.

And this is perfect for me: I can tail -f the log file and then use log.Printf whereever I want.