New features for audio

[marcvanopstalndla]

At NDLA.no we would like to have some new features in the audio component. Proposed updates:

• When including audio-file: setting for playing from the indicated start to the end
• When including audio-file: setting for playing from the indicated start to the indicated stop
• Player option: set the speed of the audio playback (standard 1x)
• Player option: fast forward and fast backward control with a customisable timeframe when including the audiofile (standard 15 seconds) Some of these featusers are important for "universal design" (UN Convention on the Rights of Persons with Disabilities) of the component.

[paulmyatt]

I would also like it to be possible to enable the standard audio player features (that have been disabled in the code): nodownload noplaybackrate.