Allow log and error functions to be specified as options.

So, if you make this change, then you can use gulp-iconfont like this:

```javascript
gulp.task('iconfont', function() {
    gulp.src(['src/iconfont/*.svg'])
        .pipe(iconfont({
            fontName: 'iconfont',
            log: function(){}
        }))
  ...
```

without needing to make any changes to gulp-iconfont itself.