Adjust versioning from `X.Y.Z` to `vX.Y.Z`

[ja-he]

From what I understand this is the format go expects for version tags. Right now using e.g. go install github.com/ja-he/dayplan@0.1.1 (works but) gives the output

go: downloading github.com/ja-he/dayplan v0.0.0-20211228224011-9645c7936e80
                                         ^~~~~~
                                         look here

Annoying, so switch.