Report summary statistics

For example,

```
A total of 1658 project instances were built from 138 projects.

Most-repeated projects:
* X.csproj
 * TargetFramework = net47
 * TargetFramework = netcoreapp2.1
 * TargetFramework = netstandard2.0
* Y.csproj
 * Configuration = Release
 * Configuration = Debug
```