replacing src with object

Sometimes you want to include other files into metadata, like cover
images or subtitles. This allows optional inclusion with the default
format header
changing write to take file appends as a data key

the write method data parameter now takes `_append: Array` as an
optional parameter, appending the files as streams to the main file (in
order)
Updating README.md
Merge pull request #3 from nucleardreamer/master

Add support for attaching additional "streams" (e.g., cover art image)