cmake package config

[daminetreg]

Dear @mrange,

First thank you for the awesome work you did on cpplinq.

I'm sending you this PR as I'm currently integrating cpplinq in a project which is based on CMake and Hunter.

The changes I brought here are only to provide a cmake config file when one install the library, it allows then to download and install the library automatically with the hunter package manager, as :

hunter_add_package(cpplinq)
find_package(cpplinq 20150908 REQUIRED)
include_directories(${cpplinq_INCLUDE_DIRS})

This is possible with the patched hunter here, but I'm doing a PR also to the official hunter which would then allow downloading any new official release made on github of cpplinq automatically.

I hope this change can be accepted, I'm available for any changes and improvements naturally. :smile: Cheers,