devel/rubygem-extpp: Add rubygem-extpp 0.1.0

Ext++ is a Ruby extension that provides C++ API for writing Ruby extension.

You can write your Ruby extension easier than Ruby's C API. Because Ext++'s C++
API reduces duplicated code needed for Ruby's C API.

You can use all Ruby's C API without any adapter layer. Because you can use C
API directory from C++.