Allow overriding the output root with a command line flag

[mythmon]

This can be helpful if you are building in an environment that is different than the configuration file expected. It can also be used to build a project multiple times to multiple directories, such as to run diff on the output to see the effect of a change.

It would however make any "custom" scripting between build and deploy harder, since the output root is no longer as predictable. Perhaps the recommendation here should instead be to make the config file read an environment variable for its output directory?