Update d3-funnel-charts.js

Hi! 

Thank you for a nice project. It helped me out a lot.

I added some stuff to your funnel code that you and others might find useful too so if you want you can use it in your project.

I added optional custom colors to the project, callback capability and the option to make the trapezoids of equal size.

Sample use:
function FunnelCallback(inputString) {
            switch (inputString) {
                case 'data 1':
                    DoStuffs1();
                    break;
                case 'data 2':
                    DoStuffs2();
                    break;
                case 'data 3':
                    DoStuffs3();
                    break;
                case 'data 4':
                    DoStuffs4();
                    break;
                default:
                    alert('Funnel callback not matching input');
                    break;
            }
        }

function MakeAFunnel() {
            var data = [['data 1', 1711, '#4c689f', FunnelCallback], ['data 2', 171, '#a7d04c', FunnelCallback], ['data 3', 51, '#4cbde8', FunnelCallback], ['Data 4', 11, '#9cd8ef', FunnelCallback]];
            var chart = new FunnelChart({
                data: data,
                width: 450,
                height: 550,
                bottomPct: 1 / 3,
                equalSizedFunnelSteps: true
            });
            chart.draw('#funnelContainer', 2);
        }

This is my first pull request so I apologize something does not make any sense for some reason...

Best,
Peter.