Exposing TagLib::PropertyMap values via C bindings

[Matt Layher]

Hello!  I'm working on a project in Go which is making use of C bindings 
for TagLib.  I am able to fetch all standard tags just fine (Artist, 
Album, Title, etc.), but it would be incredibly useful to be able to 
fetch an arbitrary tag from the PropertyMap via the bindings.

For example, even though "ALBUMARTIST" is not supported as a standard 
tag of TagLib, I understand that it may be available via the 
PropertyMap.  If I were able to call a function, specify "ALBUMARTIST" 
as the tag I want from the property map, and have the raw value 
returned, it would be incredibly useful.

Unfortunately, I am not familiar enough with C++ to be able to implement 
this functionality myself.  If possible, could I request that a function 
be added to the C bindings to enable this functionality?

Thank you very much for your time, and I appreciate all of the hard work 
that you have done to make TagLib truly great.

- Matt Layher