bob config fixes

The current incarnation of bob config does not allow one to remove variables from the configuration file.

A subcommand to unset a variable would be welcome - the command should also allow for the file to be cleared of all variables starting with a substring. E.g.:

$ bob config unset bob.db.atnt #removes everything concerning bob.db.atnt

Other than that, it would be beneficial if the documentation of this command followed the changes in #61 (closed) (i.e., documenting options and arguments via the epilog and help entries, followed by examples.