Add function for seeking time in iOS control center and lockscreen

[BuddyLReno]

This commit adds support for scrubbing with the progress bar on iOS' control center and lockscreen. This fixes #60 and #61.

Ionic Native PR: danielsogl/awesome-cordova-plugins#1727

• New event: music-controls-seek-to
• New function: MusicControls.updateElapsed(args: object);

THIS IS A BREAKING CHANGE (probably needs to update to 2.0.0 with this)

• Change: messages are now returned as json and need to be parsed.
  JSON.parse(action) === { message: "music-controls-next" }

This is due to the fact that we need to get the new position of the audio file after the user has scrubbed with the iOS scrubber. Seconds are returned by iOS:

JSON.parse(action) === { message: "music-controls-seek-to", position: "208" }
The position param returned by the music-controls-seek-to event is in seconds.

[homerours]

Thank you @BuddyLReno ! It looks good :) (However, I have no iOS device to test it...)
The Json messaging system was a needed evolution, everything is fine.

[BuddyLReno]

No problem @homerours! I've been using the new function with success in my company's iOS app streaming mp3's from libsyn. Should the version number be bumped? If someone updates to this version of the plugin, their implementation will break.

[BuddyLReno]

Also haven't heard from Ionic about the ionic native PR I placed at the same time to add this function.

[homerours]

Ok great! I going to update the version number. And if in the future you would like to add some more updates, I can grant you the "writing rights" on this project.

[BuddyLReno]

@homerours Totally! I'd love to continue to contribute. 👍