Allows defining a list of supported audio formats, and allows
switching on and off DoP with certain formats.

This is a first rough draft.  The setting syntax and its semantics may
still be redesigned.

Closes #107

Add API documentation
Support 64 bit offsets
Use InputStream for all reads

Closes #43

Preparing the protocol spec for multi-player support.

The first step to multi-player support.  Not much, just a dummy
command.

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

These days, this is common knowledge, and doesn't need a lengthy
explanation.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

https://bugs.musicpd.org/view.php?id=3876