Add support for overriding target frameworks.

[gibbed]

This allows override the TargetFrameworks that Tommy uses, if you include the project itself in your solution, via adding something like this to a parent directory's Directory.Build.props:

<Project>
  <PropertyGroup>
    <Tommy_TargetFrameworksToBuild>net48</Tommy_TargetFrameworksToBuild>
    <TommyExtensions_TargetFrameworksToBuild>net48</TommyExtensions_TargetFrameworksToBuild>
    <TommyExtensionsConfiguration_TargetFrameworksToBuild>net48</TommyExtensionsConfiguration_TargetFrameworksToBuild>
  </PropertyGroup>
</Project>

For the main project, it will try:

• Tommy_TargetFrameworksToBuild
• TargetFrameworksToBuild

Otherwise defaults to the original TargetFrameworks.

For the extensions projects, it will try:

• TommyExtensions_TargetFrameworksToBuild/TommyExtensionsConfiguration_TargetFrameworksToBuild
• Tommy_TargetFrameworksToBuild
• TargetFrameworksToBuild

Otherwise defaults to the original TargetFrameworks.

[dezhidki]

Seems fine by me, thanks!