Update to the new project system

[sharwell]

• Migrate to the new project system, with no functional changes to the build outputs
• Include a proper .NET Standard 1.0 binary

[Knagis]

Thank you for this! Could I add you as a collaborator on the project so my lack of time wouldn't be an obstacle for merging it when it is ready?

[sharwell]

This seems to be a good start. Additional work can be done in follow-up changes:

• The manual .nuspec could be changed to a "create package on build"
• The .nuspec could be moved to CommonMark.Console if we want to create the package on build without worrying about the circular dependency
• The .nupkg could be attached to the build output
• The tests could be integrated with OpenCover/codecov.io for code coverage
• Nerdbank.GitVersioning could be integrated for automatic package versioning support

I wasn't sure which of these (if any) you would be interested in, so I'm pointing them out but not implementing them unless requested. 😄