NormalizeVerString returns the passed string stripped of all characters which are not valid according to the semantic versioning guidelines for pre-release version and build metadata strings.

Version returns the application version as a properly formed string per the semantic versioning 2.0.0 spec (http://semver.org/).

AppPreRelease MUST only contain characters from semanticAlphabet per the semantic versioning spec.

appBuild is defined as a variable so it can be overridden during the build process with '-ldflags "-X main.appBuild foo' if needed.