Target .NET Core 2.0, enable SourceLink

[qmfrederik]

This PR contains a couple of improvements to bring the package up to date:

• Target .NET Standard 2.0 (part of .NET Core 2.0). This simplifies the number of dependencies required on .NET Core.
• Use a PackageLicenseExpression to indicate the package uses a MIT license
• Use NerdBank.GitVersioning to auto-generate a version number, which increases with every commit
• Use SourceLink and publish source packages to NuGet

Let's see if CI likes this.

[qmfrederik]

@brewdente Not sure why CI isn't triggered here, but CI passes on my fork: https://ci.appveyor.com/project/qmfrederik/chromedevtools/builds/24907354