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

[ian]

Reviewers: Neil Puttock,

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 manipulating semi-documented parser variables.

ignore .gitignore

Please review this at http://codereview.appspot.com/144047

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