Add ability to override how properties are initialized

[bradparks]

I wanted to be able to use text variables names as the actual variable values, and am suggesting the following simple way of doing so - by adding a callback that can replace the prop function.

Example usage:
This example will return true or false, if the search string matches the input text.

e.g.
"hi and there and (this or dog)" => returns 1
"hi and black and (this or dog)" => returns 0

  var testString = "hi there how are you?\nthis is another day";

  function mapIt(obj, name)
  {
    var result = testString.indexOf(name) != -1;
    return result;
  }

  try {
    var searchFcn = compileExpression(expression, additionalFunctions, mapIt); // <-- Filtrex!
    input.css('background-color', '#dfd');
  } catch (e) {
    // Failed to parse expression. Dont search
    input.css('background-color', '#fdd');
    return;
  }