Add CMake target for embedding the Python interpreter

All targets provided by pybind11:

* pybind11::module - the existing target for creating extension modules
* pybind11::embed - new target for embedding the interpreter
* pybind11::pybind11 - common "base" target (headers only)

tests/test_cmake_build/embed.cpp

@@ -0,0 +1,30 @@
+#include <pybind11/pybind11.h>
+#include <pybind11/eval.h>
+namespace py = pybind11;
+
+PyObject *make_module() {
+    py::module m("test_cmake_build");
+
+    m.def("add", [](int i, int j) { return i + j; });
+
+    return m.ptr();
+}
+
+int main(int argc, char *argv[]) {
+    if (argc != 2)
+        throw std::runtime_error("Expected test.py file as the first argument");
+    auto test_py_file = argv[1];
+
+    PyImport_AppendInittab("test_cmake_build", &make_module);
+    Py_Initialize();
+    {
+        auto m = py::module::import("test_cmake_build");
+        if (m.attr("add")(1, 2).cast<int>() != 3)
+            throw std::runtime_error("embed.cpp failed");
+
+        auto globals = py::module::import("__main__").attr("__dict__");
+        py::module::import("sys").attr("argv") = py::make_tuple("test.py", "embed.cpp");
+        py::eval_file(test_py_file, globals);
+    }
+    Py_Finalize();
+}