Create branch line_intersect

创建线段相交branch及提交Readme
Draw two line in canvas
Rewrite drawing lines logic

重写绘制线段逻辑，支持鼠标左右键分别绘制
Add line intersect algorithm

添加线段相交算法
Update README.md

更新README说明下使用方法（P.S. git的diff不对啊！）
Merge pull request #1 from Illvili/line_intersect

Line intersect merged