add nuget

[smanders]

building clang-format VS extension with VS 2017 requires nuget.exe "to pull in required assemblies"
see issue #189 (comment)

[smanders]

for now I plan to only use nuget internally (for llvm/clang/clang-format/clang-format-vs to build the clang-format Visual Studio extension), but I committed an untested use script and the cmake to copy the executable to the staging directory, so that it would be easy to install/distribute the nuget executable with an externpro install

externpro/projects/nuget.cmake

Lines 26 to 39 in 23fae80

	if(DEFINED XP_NUGET_COMPLETE_PKG)
	xpGetArgValue(${PRO_NUGET} ARG VER VALUE VER)
	configure_file(${PRO_DIR}/use/usexp-nuget-config.cmake ${STAGE_DIR}/share/cmake/
	@ONLY NEWLINE_STYLE LF
	)
	ExternalProject_Add(nuget_bld DEPENDS ${tgtDep}
	DOWNLOAD_COMMAND "" DOWNLOAD_DIR ${NULL_DIR} CONFIGURE_COMMAND ""
	SOURCE_DIR ${NULL_DIR} BINARY_DIR ${NULL_DIR} INSTALL_DIR ${NULL_DIR}
	BUILD_COMMAND ${CMAKE_COMMAND} -E copy_if_different ${DWNLD_DIR}/${fn} ${STAGE_DIR}/bin/${fn}
	INSTALL_COMMAND ""
	)
	set_property(TARGET nuget_bld PROPERTY FOLDER ${bld_folder})
	set(tgtDep nuget_bld)
	endif()

this is somewhat similar to what I did for yasm -- I don't install the use script by default
https://github.com/smanders/externpro/blob/18.04.1/projects/yasm.cmake#L31-L37

[smanders]

completed with commits to dev branch referenced above