Add ability to change 'v' prefix when tagging with `npm version`

Currently, `npm version <semver>` (or its variants) increases package's version, `git commit`, and then `git tag` it as `v{version}`. Although, some projects (eg. [jquery](https://github.com/jquery/jquery), or [backbone](https://github.com/documentcloud/backbone)) don't use the `v{version}` pattern to tag its releases. This change allows one to customize the "v" prefix as described below.

In the command line:
```
$ npm version <semver> --version-prefix="<prefix>"
```

In the `packages.json`:
```
  versionPrefix: "<prefix>"
```

Although this is not a critical issue (or not even an issue at all), it would be a good thing to have.