Add function overloading for typings in getUserPlaylists - Fixes #135

[JMPerez]

The getUserPlaylists function checks if the first parameter is a string, in which case it uses it as the userId, otherwise it assumes the request is for the current user and considers the input parameters as options and callback.

Passing undefined as userId assumes that the first parameter is options, but the code removes the first parameter.

Typescript supports the definition for function overloading, which is what's needed here https://www.typescriptlang.org/docs/handbook/functions.html#overloads

[coveralls]

Coverage remained the same at 98.396% when pulling 16432e5 on getUserPlaylists-typings into 0fa940e on master.