Timestamp Arguments

[xloem]

This PR adds a command line argument to bug creation, commenting, and status changing, that lets one change the timestamp from the present.

The purpose is to facilitate using git-bug inside single-use scripts to import issues from archives, addressing the reason I opened issue #489 .

The changes are minor, but I have never coded in go before, and mostly copied structures from elsewhere in the repository to accomplish this.

[6543]

I would not just add option to pass unix timestamp. Instead use a time parsing lib to be able to use all kinds of time arguments.

Also we dont need other func we are no lib, just refactor the code