Spotify Web Api Node Versions Save

• Fix #349: Transfer playback options parameter is optional.

• Bug fix. Thanks again @yamadapc.

• Modified functions that operate on playlists to drop the user id parameter. This is a breaking change. https://github.com/thelinmichael/spotify-web-api-node/pull/243
• Updated superagent to fix a security warning. https://github.com/thelinmichael/spotify-web-api-node/pull/211
• Fixed a bug by which an empty user was not handled properly in getUserPlaylists(). https://github.com/thelinmichael/spotify-web-api-node/pull/244

• Modernized stack for a better developer experience. Integrated prettier and jest. This simplifies the amount of dev dependencies.
• Improved calls to save and remove saved tracks by adding a key as specified in the Spotify docs (See PR). Thanks to @yanniz0r and @adcar for bringing it up.

• Added support for seeking and setting volume. Thanks to @isokar, @jamesemwallis, @ashthespy, and @vanderlin for your PRs.

New features

• @DalerAsrorov added support for uploading a custom image to a playlist in this PR.
• You can now pass a device_id when playing and pausing playback. @pfftdammitchris started a PR to add device_id to the play() method. The changes served to another PR where we included the functionality. Thanks!

Documentation

• Added documentation in the README for getMyCurrentPlaybackState(). Thanks @PanMan for your PR!

Internals

• @brodin realized we there was a lot of duplicated code and refactored it in a great PR.

• Change README to reflect new authorization. Thanks @arirawr for the PR.
• Add support for 'show_dialog' parameter when creating authorization url. Thanks @ajhaupt7 for the PR.
• Add support for playback control (play, pause, prev, next), shuffle and repeat. Thanks @JoseMCO for the PR.
• Add support for currently playing. Thanks @dustinblackman for the PR.
• Fix to remove unnecessary deviceIds parameter from request to transfer playback. Thanks @philnash for the PR.

• Change addTracksToPlaylist to pass the data in the body, preventing an issue with a long URL when passing many tracks. Thanks @dolcalmi for the PR
• Add support for fetching recently played tracks. Thanks @jeremyboles for the PR.

Use encodeURIComponent instead of encodeURI to encode the user's id. 'encodeURI' wasn't encoding characters like / or # that were generating an invalid endpoint url. Thanks @jgranstrom for the PR.