Checking for use of Web Audio API

howler.js already detects whether it can use the Web Audio API.  This adds a global method that returns a boolean, providing access to that info.

Maybe it would make more sense to use a read-only global property instead?  Also, I debated on what to name the method, still not sure which is best:

webaudio
webAudioAPI
usingWebAudio
usingWebAudioAPI