Add support for dingtalk

[yifeikong]

Dingtalk is quite popular in China, please consider supporting it. Thanks!
docs in Chinese: https://ding-doc.dingtalk.com/doc#/serverapi2/al5qyp

[welcome]

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[liiight]

I would very much like to do so but I'm afraid I'll need English documentation to do so. Is there an English translation of the docs?

[yifeikong]

I'm afraid that there is no English version of these docs. I will see if I could find some time to submit a PR. It seems that adding a dingtalk.py in notifiers/providers directory should do the trick.

[liiight]

It's less of a question of where to put the file and more of what code it should contain 😉

I'll try to see if google translate can help

[blackstrip]

PR: #431