Add back implicit configuration and target framework defines

[Enrico Sada]

By default:

• a configuration like debug add DEBUG define
• a target framework like netcoreapp1.0 add NETCOREAPP1_0 define

Only the configuration defines can be disabled with $(DisableImplicitConfigurationDefines) property set to true

[Enrico Sada]

done with #77 by @neoeinstein