Adds Pre-Processing feature. Related issue #40

[3F]

Closes #40

After serial questions about manipulation for modules together with our tool and some misunderstanding such for "How do I do this without the /ndebug flag" https://twitter.com/GitHub3F/status/1248621686930337795

I decided to add configurable Pre-Processing where we can configure like ILMerge, Conari, and others.

"WIP" since PackageReference is missing copy files to output if .NET Core/SDK-style

• https://github.com/NuGet/Home/issues/4837

But GeneratePathProperty feature https://github.com/NuGet/Home/issues/4837#issuecomment-474184722 will require additional processing to find related platform:

<PackageReference Include="Conari" Version="1.4.0" GeneratePathProperty="true" />
...
<Copy SourceFiles="$(PkgConari)\lib\netstandard2.0\Conari.dll" DestinationFolder="$(OutDir)" />

Thus CopyLocalLockFileAssemblies still is more suitable for this case. But I'm still not sure, I'll try to review this later.

[3F]

Well, CopyLocalLockFileAssemblies is here; Actually I don't want to see all dependencies inside output folder because it can be confusing. But ...

+Pre-Proc feature has been splitted into separate PreProcGear implementation (IProjectGear)

[3F]

Rebased onto 2131353e5c