Add CodeCov badge /s code coverage on CI

[Athari]

Add CodeCov badge. As a bonus, it shows pretty diagrams of code coverage.

Use example CodeCov C# repository as an example.

Code coverage apparently requires debug(?) build with full non-portable PDBs, so switch to Build Debug -> Test & Coverage -> Build Release -> Deploy sequence? There're no parallel builds on free AppVeyour plan, so no point in separating build jobs even if it's possible. Symbol Packages for .NET Standard libraries are probably supposed to contain portable PDBs, so there's no way around double build, I guess.

Could also be a good time to find out why the heck only 1 set of tests is executed on AppVeyor instead of 6 ( { XAML, No-XAML } x { Core 1.1, Core 2.0, .NET 4.6 } ).

Related:

• Support portable pdb files OpenCover/opencover#610
• .net core support OpenCover/opencover#595
• .net core support OpenCover/opencover#595 (comment)

P. S. Coveralls to CodeCov is like SensioLabs Insight to Scrutinizer. Don't bother with it. Not worth the badge. Well, maybe. If it's one line of code in the build script.