Feature for examining audio block content

[inker610566]

Thanks for the great work of crafting matroska format parser for golang. The library is currently used to verify MediaRecorder from our team. Now, I'm adding test for verifying whether the result mkv from MediaRecorder is muted. Just curious about whether the library support examining the content of simple block of audio track. I saw the information parsed from library can identify those audio blocks but the data field here: https://github.com/pixelbender/go-matroska/blob/7953d3f318cca6843ea20a6655b73708d168d748/matroska/matroska.go#L137 gets comment out. I wonder if it's for fixing the OOM issue. In that case, can we try to add lazy loading mechanism like adding offset and size fields of data to block then gets their value from Reader of Block's UnmarshalEBML method. With information of these 2 fields, the user can easily read the data after decoding.