Feature Request: ability to specify custom filename for midi output

[Javier Ruiz-Alma]

Requesting adding the ability to state a filename for midi output from within 
score's midi block (or other score-specific block).

\book{
  \bookpart{
      ...some markup
      \score{
         .....music for ScoreOne
         \layout{}
         \midi{ \set midifilename = "chap1-piece-one" }
      }
      ...more markup
      \score{
         .....music for ScoreTwo
         \layout{}
         \midi{ \set midifilename = "chap1-piece-two" }
      }
   }
  \bookpart{
      ...more markup
      \score{
         .....music for ScoreThree
         \layout{}
         \midi{ \set midifilename = "chap2-piece-one" }
      }
      ...more markup
      \score{
         .....music for ScoreTwo
         \layout{}
         \midi{ \set midifilename = "chap2-piece-two" }
      }
   }
}

Allows one to build a single book.ly file (collection of scores) that produces 
a single "book.pdf" output, as well as individual midi files with user-
selected names significant to their content/context (i.e. chapter number, 
composer, opus, etc.).