Add pass-through parameters for embeds

Add a way for the developer to add parameters to the embed URL or the
iframe from the tag. This allows the developer to specify advanced YouTube
or Vimeo options like YouTube's 'controls' option or Vimeo's 'color'
option, or even the 'wmode' option for an iframe.

{videolink-field:embed youtube:controls="2" vimeo:color="#4488ff"}

Fixes #7