Add new syntax for time spent in issue

[Joshuaalbert]

It would be mighty useful if our developers could just do something like,

estimate 5
spent 2020-04-03 1

Sometimes when a revision to estimate is made, because of an obstacle, we have to go back and modify each prior remaining entry, which is tedious, when it would be so much simpler to just change the estimate entry.
We like to keep our story point system linear, so if something requires more/less work, then we increase/decrease the estimate.

Note: this would be backwards compatible with the remaining syntax as long as the two are not mixed in the same issue.

[Joshuaalbert]

Sorry, realised this issue was already created (and I even commented on it before and forgot).