New functions: seekAndCenter, toggleScroll. Implemented regions. Marks g...

- New functions:
   - seekAndCenter 
   - toggleScroll

- Removed handlerSize from parameters.
- Fire selection-updated with selection clear with null parameter.
- Mute while seeking paused to avoid noise.
- changed handler elements to handlers DOM tag with wavesurfer_handler class.
- Fixed bug where marks had handlers.
- Added wavesurfer parameter to drawer.
- Added regions with events (region-created, region-in, region-out).
- Added mark-updated, mark-removed events.

- Updated readme