Add timestamp flag for request, comment, accept

I propose to add an optional `-timestamp` flag to the following commands:

- `git appraise request`
- `git appraise comment`
- `git appraise accept`

If the flag is not specified, it will default to the current timestamp. The timestamp will then be used for the request and comment JSON objects.

This will make it easy to script importing historical code reviews from other systems. No special code will be required, just normal shell commands. (User can be easily set in the shell with `git config`.)