Allow timestamping to be per signature configuration.

[priteshbandi]

One signer implementation(like file-based one) can be reused by multiple organization. Some organizations might not want to timestamp signature because of availability concerns or something else but other might prefer to timestamp signature.
So there should be a way to for user to disable/enable time-stamping on per signature basis. One way to achieve is that only add timestamp when user explicitly specifies it

• notation sign --timestamp=${serverURL} <-- signs and timestamps
• notation sign <-- only signs

Originally posted by @priteshbandi in #11 (comment)