Add diff subcommand

[fujiwara]

Add diff subcommand. This command shows a diff of function.json compared with the latest deployed function configuration.

$ lambroll diff
2020/07/01 15:32:21 [info] lambroll v0.7.0
--- arn:aws:lambda:ap-northeast-1:123456789012:function:test
+++ function.json
 {
   "Description": "test",
   "FunctionName": "test",
   "Handler": "index",
-  "MemorySize": 512,
+  "MemorySize": 1024,
   "Role": "arn:aws:iam::123456789012:role/test_lambda_role",
   "Runtime": "go1.x",
   "Tags": {},
   "Timeout": 30,
   "TracingConfig": {
     "Mode": "PassThrough"
   }
 }

2020/07/01 15:32:21 [info] completed