Support launching .NET apps (source and compiled) using dnx

Now that the `dotnet dnx` command and `dnx` script are in the SDK, we should update them to support launching .NET apps other than tools, including from source (project files or file-based apps), and assemblies. This makes `dnx` a universal launcher for .NET apps whether they be:

- `dnx MyApp.dll`, `dnx ../path/to/MyApp.dll`
Argument ends with *".dll"*
This would dispatch to `dotnet MyApp.dll`, `dotnet ../path/to/MyApp.dll`
- `dnx ProjectFile.csproj`, `dnx ../path/to/ProjectFile.csproj` (and other known project file types)
Argument ends with *".csproj"*, *".vbproj"*, *".fsproj"*
This would dispatch to `dotnet run --project ProjectFile.csproj`, `dotnet run --project ../path/to/ProjectFile.csproj`
- `dnx app.cs`, `dnx ../path/to/app.cs`, `dnx app` (extensionless source file)
Argument ends with *".cs"*, or is path to existing extensionless file
This would dispatch to `dotnet run app.cs`, `dotnet run ../path/to/app.cs`, `dotnet run app`