Enable tree shaking for echarts

At the moment, the entire echart module is included, also if only some components are needed.

I use the library in a side project of mine, and I use only the line graph - I don't think I need the entire echarts library.

Importing only the components used by every single type graph would help webpack tree shaking the bundle.

The guide suggests to do

// include ECharts main module
var echarts = require('echarts/lib/echarts');
// include bar chart
require('echarts/lib/chart/bar');
// include tooltip and title component
require('echarts/lib/component/tooltip');
require('echarts/lib/component/title');

instead of

var echarts = require('echarts');