Add CMake support to include it easily as git submodule

[tfabbri]

At the moment to use this library as submodule into an existing cmake project, it requires the use of ExternalProject_Add. It will be useful to add the support to cmake.

[aminroosta]

@tfabbri Good idea, are you willing to send a PR :-) ?

[tfabbri]

I'll try to have a look into that.

[epsilon-phase]

It doesn't take that. This is header only, except the tests. Just set it as an include directory. It works fine.

[v1993]

@aggsol I'm probably too late, but you can check out my project's CMake which use this great library: https://github.com/v1993/generators/blob/master/markovSQLite3/CMakeLists.txt.