Migrate to dotnetcore

[greatkeke]

我迁移到了dotnet core平台上，基于netstandard2.0。

希望可以新建一个dotnetcore分支。

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 3 committers have signed the CLA.

:white_check_mark: kexxxfeng
:x: keke-cnblogs
:x: cnblogs-dudu
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