consider using `discid_read_sparse` when/if implemented in libdiscid

[JonnyJD]

http://tickets.musicbrainz.org/browse/LIB-29 is quite new and probably still takes some time.

However, if we include that together with the ISRC API addition, then we might circumvent performance problems later on.
So everybody who only wants the id can use read() now and we will make simple reads faster when LIB-29 is implemented.
Later we can introduce an optional mode option where you have to tell if you want ISRCs and the "ID users" don't have to change anything.

[JonnyJD]

This still needs decision for how to use the mode option.

Bit masks aren't any good for a clean python interface, or are they?

String lists would have the advantage, that we can just pass FEATURES when we don't care about performance and want everything. The feature strings are stable (taken from discid.h).

[JonnyJD]

Implemented in c998986 (#10).

There is now an optional features parameter that accepts a list of strings.