Export the HTMLlint module or provide an API for customization

[paulsouche]

Not an issue but it should be great if we could do this

var gulp       = require('gulp'),
      htmlhint = require('gulp-htmlhint');

  htmlhint.addRule({
     //my rule : see https://github.com/yaniswang/HTMLHint/issues/47
  });

  gulp.task('htmlhint', function() {
    gulp.src('views/**/*.html')
      .pipe(htmlhint('.htmlhintrc'))
      .pipe(htmlhint.reporter());
  });

I think this could be easely done. See #13

Thanks