feat(changelog): add optional versionFile option

- add option 'versionFile' to specify a file which should contain a json (like package.json or bower.json) with a 'version' property

I think this is pretty useful, because currently the changelog task just expects a pkg object which does not nessecary exist. So you can now say that the version number is defined in e.g. bower.json rather than a maybe existing pkg object which normally comes from a parsed package.json.