provide access to warnings as objects (see issue #14)

HI Paul, I have implemented issue #14 as far as I was able to. It boils down to a new method in ABCJS.parser.getWarningObjects which yields an array of objects

  {
   message:str,       // the error message 
   line:line,              // the broken line
   startChar: multilineVars.iChar + col_num, // the startChar of the error
   column: col_num  // the position of the error within line
 }

Based on this, i was able to highlight the error position in my editor.

As this API does not break backwards compatibility at all, I kindly ask you to incorporate it.