Make configuration files optional

[davidbrochart]

Now that we can pass configuration at the command line, I guess that configuration files become optional.

[coveralls]

Coverage remained the same at 97.26% when pulling 87714a5 on davidbrochart:optional_yaml into b651db8 on asphalt-framework:master.

[agronholm]

I'm not sure about this. Is this really practical?

[davidbrochart]

I think yes. I have a project where I use Asphalt, and it can run with no configuration file (the only thing it needs is the component.type).
But more generally, I don't see why configuration from file would be required, since you can also configure it through the command line.

[agronholm]

Alright, I don't see any real harm from this.