mermaid中的甘特圖
甘特圖是一種條形圖，常用於說明項目進度表。甘特圖說明了項目的終端元素和摘要元素的開始和結束日期。
示例：
gantt
title A Gantt Diagram
dateFormat YYYY-MM-DD
section Section
A task :a1, 2014-01-01, 30d
Another task :after a1 , 20d
section Another
Task in sec :2014-01-12 , 12d
another task : 24d
1. 句法
gantt
dateFormat YYYY-MM-DD
title Adding FANTT diagram functionality to mermaid
section A section
Completed task :done, des1, 2014-01-06,2014-01-08
Active task :active, des2, 2014-01-09, 3d
Future task : des3, after des2, 5d
Future task2 : des4, after des3, 5d
section Critical tasks
Completed task in the critical line :crit, done, 2014-01-06,24h
Implement parser and jison :crit, done, after des1, 2d
Create tests for parser :crit, active, 3d
Future task in critical line :crit, 5d
Create tests for renderer :2d
Add to mermaid :1d
section Documentation
Describe gantt syntax :active, a1, after des1, 3d
Add gantt diagram to demo page :after a1 , 20h
Add another diagram to demo page:doc1, after a1 , 48h
section Last section
Describe gantt syntax :after doc1, 3d
Add gantt diagram to demo page :20h
Add another diagram to demo page:48h
2. 日期格式化
具體參考：http://momentjs.com/docs/#/parsing/string-format/