Feature: Defer youtube player lib loading

[zwacky]

Hey there, great library! The goal of this PR is to give the developer, if they want, the control of when the player api lib will be loaded.

e.g. currently whats being loaded right away:

• player_api (1kb)
• www-widgetapi.js (9kb)
• per loaded video, not playing
  • embed html (17kb)
  • js/bg (5.3kb)
  • player css (40.4kb)
  • embed-player.js (39kb)
  • base.js (1.2mb)
• doubleclick ad trackings:
  • id (0.2kb)
  • ad_status.js (0.01kb)

Bandwith and parse time is relevant here.

Tell me what you think, cheers!