Hint ':add' for command start

Analogous to
```
timew start FOO BAR BAZ
stop FOO
```
which only would stop tracking for `FOO` but will keep tracking running for `BAR` and `BAZ`, add a hint `:add` such that
```
timew start FOO
timew start :add BAR
```
would stop tracking for `FOO` and then create a new interval with tags `FOO BAR`.

Start by tracing how the `timew start` command parses tags and replaces the active interval. Compare the existing `start FOO BAR BAZ` behavior with the requested `start :add BAR` behavior. Done means `:add` preserves the current tags and creates the combined `FOO BAR` interval, with coverage for the command's existing start behavior.

Written by the indexing model from the issue text.