Set properties dynamically (useful for lyrics, composer etc)

[xidb]

Hello.
Superb lib you have here, it helped me a lot.
But it lacks ability to set custom tags, like lyrics.
So i removed all code, that was implicitly setting things like album artist or bpm and added a piece, where those things saved dynamically. It checks and skips if current iteration is not an common tag (artist, title).
I checked for .mp3, .m4a files and i can confirm that lyrics added that way are showing up in iTunes or other software (ffmpeg, mediainfo, tagscanner).
I think it will also with an issue you have where there was a question about grouping field.

[xidb]

I also made a support for UTF filenames.
Also i consider removing from readTagsSync all properties, that were implicitly set to string in resulting object and set them dynamically if they are found in tag's property map.
So, for example, BPM will stay implicit, as it's an integer, but i remove implicit COMPOSER and will set it dynamically.
It could be a bad idea? =)

[kbuffington]

@xidb Your PR looked like something I might be interested in as I need dynamic properties as well, but when I try and build this on OSX, I get the same errors the Travis CI build does. Did you ever do more work on this?