Tag command: add Semver pre-release support

Similar to #3 the `tag` command should support `pre-releases`.

**Examples**

```sh
~> tag --prerelease
# v2.1.1-0
~> tag --prerelease
# v2.1.1-1
~> tag --premajor
# v3.0.0-0
~> tag --preminor
# v3.1.0-0
~> tag --prepatch
# v3.1.1-0
~> tag --prerelease
# v3.1.1-1
```
