Add an option to reorder the button groups

This PR replaces that one: https://github.com/toopay/bootstrap-markdown/pull/42.
The feature and especially the associated code is much more simple and clean.

The new option `reorderButtonGroups` give the possibility to reorder the button groups according to our needs.

E.g.:
```js
reorderButtonGroups: ['groupFont', 'groupCustomLink', 'groupMisc', 'groupUtil']
```