PR 572: Add initializer list constructors to Matrix and Array (include unit tests and doc)

- {1,2,3,4,5,...} for fixed-size vectors only
- {{1,2,3},{4,5,6}} for the general cases
- {{1,2,3,4,5,....}} is allowed for both row and column-vector

doc/snippets/Matrix_initializer_list2_cxx11.cpp

@@ -0,0 +1,3 @@
+Matrix<int, 1, 6> a {1, 2, 3, 4, 5, 6};
+Matrix<int, 3, 1> b {1, 2, 3};
+cout << a << "\n\n" << b << endl;