To allow different release names other than specifically mm.vv.pp, this
adds the ability to use a prefix too.

For example, you can use:

```
$ git release --semver minor --prefix "prefix-"
```

This would allow the previous tags to be in the form `prefix-0.1.0`. A
more obvious use case for this would be a single letter, such as `v` or
`r` to use tags like: `r0.1.0` or `v0.1.0`.