slt-release: change -n to -p, so `-up` updates

It should be easier to remember the options like this.

Originally I tried not to use `p`, because I thought it would be unclear whether it meant `git Push` or `npm Publish`, but now I can never remember what the option names are... I think that `-up` combination is pretty easy, though.

What think you, @bajtos and @rmg ?
Can we keep `n` as an alias for `p` in order to preserve backwards compatibility?