Adding attribute selector with variable concatenation feature

[chencaize]

What:Adding attribute selector with variable concatenation feature

Why:I use less.js to compile file , but I find that it can not handle the variable concatennation in attribute selector. This is not comfortable , and sass is support this feather , but sass' can not support something that less support.

Checklist:

• [ ] Documentation N/A
• [x] Added/updated unit tests
• [x] Code complete

[chencaize]

it can not pass one of the test,because of the puppeteer.I use the master to run this cases ,it sometimes will occur this problem.I think this is none of my code's problem,it is the puppeteer's problem,please check ,best wishes. The problem is like below: TimeoutError: Navigation timeout of 30000 ms exceeded