Add Hold Music Funlet

[imnotjames]

Adds a "Hold Music" funlet, similar to the hold-music twimlet.

Instead of accepting HTTP parameters for defining the bucket & message, this instead uses environment variables.

Closes #20

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[imnotjames]

For anyone that reviews this - should I stay away from async/await or can I rewrite some of this to use that instead? It'd be a bit cleaner than all the promise chains.

[philnash]

Hey @imnotjames, thanks for this!

Twilio Functions supports Node version 8 and 10, both of which have support for async/await so no need to hold back there!

Also, could you support both environment variables and URL parameters for the variables? We want the funlets to be drop in replacements for twimlets, so supporting those parameters is still important.

[imnotjames]

Regarding async/await , fantastic. I'll take care of that as well as supporting both mechanisms for input values.