Added possibility to add a colored min/max area for a series

[siberianbearr]

This change allows to add min/max values to a series. The renderer takes the series color as default color an modifies it a bit, alternatively a custom color can be passed.

var graph = new Rickshaw.Graph( {
	element: document.querySelector('#chart'),
	renderer: 'line',
	series: [{
		color: 'steelblue',
		data: [ { x: 0, y: 23}, { x: 1, y: 15 }, { x: 2, y: 79 }, { x: 3, y: 40 } ],
		min: 20,
		max: 50,
		limitColor: "#FF6766" 
		}]
	});

graph.render();

Produces this output:

More complex example: