Add test coverage and integrate it to Codecov.

[ygj6]

Steps for integrating coverage to the Codecov in the next branch:

1. First, you need to login to the Codecov with a github account;
   If you have logged in to the system once, skip this step.
2. Then execute the CircleCI test, it will automatically integrate the report into the Codecov.
3. Finally, in the link below, change the master branch to the next branch:
   https://app.codecov.io/gh/vuejs/vue-class-component/settings

Here's my final result:
https://codecov.io/gh/ygj6/vue-class-component

[ygj6]

@ktsn Coverage is used as the access control to ensure code quality.Coulud you please take time to review the code?
By the way, do you accept new maintainers,I am very glad to contribute.