Derek set milestone: current/next

Needs more thought - but I can't always remember what milestone number to pick when doing this on mobile for instance.

`Derek set milestone: 0.5.4`

vs:

`Derek set milestone: next`

Which could be: 0.5.5

`Derek add milestone` -> Might just add the latest one available.

There are other tools that can generate change logs but this seems like a simple feature to explore more.