Add support for Azure Pipelines

[hugovk]

With the recent changes at Travis CI, many open-source projects are taking advantage of and migrating to Azure Pipelines as a CI (it gives twice the number of parallel builds, and allows easy testing of Linux, Mac and Windows).

It would be great if Codecov supported Azure Pipelines, like Travis, without needing to set the token as a secure environment variables in the Azure Pipelines dashboard, or worrying about putting the token in cleartext in the config.

Here's a list of env vars in Travis and their analogues in Azure:

• https://docs.microsoft.com/en-us/azure/devops/pipelines/migrate/from-travis?view=azure-devops#predefined-variables

Thank you!