Provide a feature gate for font-related packages

[collosi]

It would be nice if you could produce an ascii-only build of this tool that didn't rely on freetype or fontconfig. Would make it more useful in server environments. I think a cargo "feature" is a great way to do this.

[ahcm]

I'm pondering whether it should be two tools.
One aggregating data and the other doing the actual plotting.
But I have no idea how it is already used.
On the otherhand, it might just be a feature.
But unfortunately I chose to default to graphics output.