repoinit: allow add or remove mixin types

The use case here is for a node that is created by a base feature.  Other (optional) features could add a mixin to the existing node later if they are also included in the distribution.

The proposed syntax would be:

add mixin mix:one to /thePath1
add mixin mix:one,mix:two to /thePath1,/thePath2

remove mixin mix:one from /thePath1
remove mixin mix:one,mix:two from /thePath1,/thePath2

If the specified paths do not exist, a warning is logged and nothing is changed.  Otherwise, the mixin is added (or removed) from those paths.