feat: prerelease can take an optional prefix argument

- updated dependency to semver 2.2.1 to allow usage of `prerelease`
- updated README with usage of `prerelease` and `prereleasePrefix`
- added `prereleasePrefix` option
Merge pull request #1 from ahmadnassri/update-semver-prerelease-prefix

feat: prerelease can take an optional prefix argument