Fix incrementing into and out of prereleases

Patching a prerelease promotes it to a release version
`semver.inc("1.2.0-5", "patch") === "1.2.0"`

`prerelease` increments the patch by default, and `preminor` and `premajor` work similarly
`semver.inc("1.2.0", "prerelease") === "1.2.1-0"`
`semver.inc("1.2.0", "preminor") === "1.3.0-0"`
`semver.inc("1.2.0", "premajor") === "2.0.0-0"`

it even works from other prerelease versions
`semver.inc("1.2.0-5", "preminor") === "1.3.0-0"`

Fixes #50