Add Tag Reader for FLAC Files

[ttpcodes]

I noticed issue #25 which seems to have been untouched since May 31st, 2016. I figured that since I wanted support for FLAC files anyway, that I would draft this up as a makeshift solution for retrieving tags from FLAC files.

I've left the jsmediatags.js file intact as there are still a few issues with the implementation I wrote up:

• A few tag types, such as comment and year, have different implementations in the Vorbis comment format or are not part of the specification altogether.
• The picture tag has not been implemented because it is stored in a different metadata block from the Vorbis comment.
• No testing has been added for this tag reader yet.

Hopefully these can be resolved in future commits.

The specification for the FLAC and Ogg Vorbis formats can be found here and here. As the two formats are very similar, a small rewrite should be all that's needed to address #25.

Expectations (will be marked off once unit tests are written):

• [x] Should properly identify a FLAC file from the header
• [x] Should properly load regular string metadata (Title, Artist, Album, Genre, Track)
• [x] Should load data regarding the album picture
• [x] Should properly call the error callback if the Vorbis comment does not exist or could not be loaded

[ttpcodes]

In addition to the changes I've made, I've also added a brief checklist of what I want to test and implement before declaring this as complete. If there's anything else you'd like, just let me know.

[aadsm]

@TehTotalPwnage, have you thought about applying for an internship as a frontend engineer? https://www.facebook.com/careers/jobs/a0I1200000LTGJBEA5/

[aadsm]

Thanks!!