Added configure support

Added the ability to confiugre Styl before compiling with it. This is useful if you want to use plugins. For example:

    styl: {
      compile: {
        options: {
          whitespace: true,
          configure: function(styl) {
            styl.use(stylVariables());
          }
        },
        files: {
          'public/main.css': ['styl/*.styl']
        }
      }
    },