Add magnet & mas for macOS

The `mas` role installed the command-line tool of the same name to
install Mac App Store package. The `magnet` role then uses `mas` to
install the tiling window manager from the App Store.

roles/magnet/tasks/main.yaml

@@ -0,0 +1,8 @@
+---
+- assert:
+    that: ansible_os_family == 'Darwin'
+
+- name: install app store package
+  mas:
+    id: 441258766
+    state: latest

roles/mas/tasks/main.yaml

@@ -0,0 +1,8 @@
+---
+- assert:
+    that: ansible_os_family == 'Darwin'
+
+- name: install homebrew package
+  homebrew:
+    name: mas
+    state: latest