Add support for extent param in config.

[faulkner]

The extent param is useful in cases where Cubism doesn't get the auto-scaling right.

Example usage: { "title": "Swap %", "expression": "100*sum(collectd(swap).eq(type,'used'))/sum(collectd(swap).in(type,['used','free']))", "extent": [0,100] }

If the extent param is omitted then it will fall back to auto-scaling.