[FEATURE] Function to check for validity of a prerelease identifier

[boxcee]

What / Why

Problem described in #349: An 'invalid' identifier will lead to an invalid semver. Now, there is no way to check beforehand whether an identifier is valid, except doing this:

const isValidIdentifier(identifier) => !!valid(inc(parse('0.0.1'), 'prerelease', identifier));

This is a request for a more straight-forward function, e.g. identifier(), which returns null (or false) if the identifier is invalid.

References

• Related to #349