Simplifying querying meta

[tmaly1980]

Instead of:

$post = Post::meta()
    ->where(function($query){
          $query->where('posts_meta.key', '=', 'revision')
                ->where('posts_meta.value', '=', 'draft');
    })

It would be really helpful/nice to have something like this:

$post = Post::whereMeta(['revision'=>'draft'])

[kodeine]

@tmaly1980 i'd be happy to merge the PR if you can on this.