make better integration with helm-mini

Hello. I would like to add some groups from ` helm-bufler-source` to `helm-mini-default-sources`. For example I would like to have a group with projectile's buffers in `helm-mini`. With ` helm-bufler-source` there are many duplicate buffers. I still don't know the best way to do this, maybe it would be nice to have their own source for each group. There are many sources here in ```helm-mini```, such as `helm-source-recentf helm-source-bookmarks helm-source-locate`. So I would like to stay with that, but use `bufler` as additional sources. Thank you!