add Tricle.configuration with sparkline option

Yeah, I think that Metric is the logical place for this option. We can make this method more concise though -- the only reason I couldn't use `options[:sparkline] || ...` is because it can be set to a falsey value. We can replace this 5-line method with:

```rb
def sparkline?
  options[:sparkline].presence || Tricle.configuration.sparklines
end
```

if that would make it more palatable for you.