Tracker 836: Add facility to change output file-name for a \book block

[ian]

Reviewers: Neil Puttock,

Message:
This patch is now available for review.

Description:
Tracker 836: Add facility to change output file-name for a \book block
or to set a suffix to prevent multiple files over-writing each other
during
a compilation.
This change allows user to to this via functions rather than having to
do
so by setting and defining semi-documented parser variables.

Please review this at http://codereview.appspot.com/143055

Affected files:
  M lily/parser.yy
  M ly/init.ly
  M ly/music-functions-init.ly
  M scm/lily-library.scm