参考这篇文档
http://www.csdn123.com/html/blogs/20131102/92241.htm
例子：
<snippet>
<content><![CDATA[
Hello, ${1:this} is a ${2:snippet}.
]]></content>
<!-- Optional: Set a tabTrigger to define how to trigger the snippet -->
<!-- <tabTrigger>hello</tabTrigger> -->
<!-- Optional: Set a scope to limit where the snippet will trigger -->
<!-- <scope>source.ruby</scope> -->
</snippet>
windows和linux下，通过按下ctrl+shift+alt+p来显示文件的scope
