define default for the options in the Config manager

default can be undone by the inverse of the original command:
e.g.:
noscroll, undo by: scroll
tsort, undo by: notsort
etc

Fixes #97

- inverse commands are ignored in the defaults (because this are already
the defaults)
- skipns and skipfile have already separated config options (skip_index
and skip_file) in the  Config manager