Allow setting the mini flag and the alignment of the buttons

[andrei-cimpan]

Hello,
Just installed your plugin yesterday and it looks great, but it would be useful to have two more features:

1. Allow setting the mini flag for fabs (that would probably also imply renaming the class from FabMiniMenuItem to FabMenuItem)
2. Allow setting the alignment of the menu items. Now they are somehow left aligned by setting the horizontal EdgeInsets to 8.0. But it would be useful to also enable centering them with the main Fab.

If you find these useful, let me know, and I can make a pull request.
Thanks!