Add machine_readable option to build fn

Author: Ashley Chaloner

packer.py

@@ -43,18 +43,22 @@ def __init__(self, packerfile, exc=None, only=None, vars=None,
         self.packer = sh.Command(exec_path)
         self.packer = self.packer.bake(**kwargs)
 
-    def build(self, parallel=True, debug=False, force=False):
+    def build(
+            self, parallel=True, debug=False, force=False,
+            machine_readable=False):
         """Executes a `packer build`
 
         :param bool parallel: Run builders in parallel
         :param bool debug: Run in debug mode
         :param bool force: Force artifact output even if exists
+        :param bool machine_readable: Make output machine-readable
         """
         self.packer_cmd = self.packer.build
 
         self._add_opt('-parallel=true' if parallel else None)
         self._add_opt('-debug' if debug else None)
         self._add_opt('-force' if force else None)
+        self._add_opt('-machine-readable' if machine_readable else None)
         self._append_base_arguments()
         self._add_opt(self.packerfile)