Timeline plugin

I've added a timeline plugin that renders a timeline for a WaveSurfer instance. It's based off of the discussion in PR #78 and manuelmitash's commit at https://github.com/manuelmitasch/wavesurfer.js/commit/3a906fbccd93daaaa23a19285b2dba08f6f7a5d2

I tried to incorporate your feedback from that PR. The plugin is canvas-only, currently - my local copy of the repo was out of date, and I didn't see any SVG stuff in there. I checked out the latest master from your repo, and didn't see the SVG renderer in there, either. I feel like I must be missing something obvious. If you can point me to the SVG renderer, I'll roll a version of the plugin that works with SVG.

Please take a look and let me know what you think. WaveSurfer has been great to work with - thanks for putting it together.