added pybind subrepo

Former-commit-id: 830960b4ce341fbebc953042a8b0188d24d82bd4

.gitmodules

@@ -4,3 +4,6 @@ url=https://github.com/libigl/nanogui.git
         fetchRecursiveSubmodules = true
         ignore = dirty
 
+[submodule "external/pybind11"]
+	path = external/pybind11
+	url = https://github.com/wjakob/pybind11.git