Use actual tag string in confirmation message of tag step

[cmyr]

Thanks for the crate, I'm finding this very useful!

I just ran into a tiny thing: When running cargo release tag, I get a confirmation message that shows the crates and versions that are going to be tagged, but it doesn't show the actual final tag text; e.g for me it said,

Tag
  crate-one 0.3.0
  crate-two 0.1.0
? [y/N] 

And so I wasn't sure what the actual tags would be, and had to go digging through the source. Maybe it make sense instead to do,

Tag
  crate-one-v0.3.0
  crate-two-v0.1.0
? [y/N]