Add update / upsert tags method

[miskun]

I'm dealing with images and had to write my own wrapper functions to deal with updating single tag in a file. More hassle when I have to deal with images. Would be nice to have update / upsert feature as part of this module.

[Zazama]

Added with f4da14a

Sync/Async usage:

//  Update existing ID3-Frame with new/edited tags
let success = NodeID3.update(tags, file) //  Returns true/false
NodeID3.update(tags, file, function(err) {  })