Provide a feature gate for font-related packages

[colossoi]

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.

[ahcm]

I think my other project plotxy is becoming more reasonable to use for plotting.
It might make sense to drop the plotting part altogeher, concentrating on producing the aggregate and use plotxy for plotting.