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_list_cxx11.cpp

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