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`.
Although Github reserves the 'visionmedia' user space(a visit to there
will be redirected to 'tj'), it's better to
rename the links from visionmedia/git-extras to tj/git-extras