fix logger reset for the shell plugin

[galdor]

The shell plugin has to reset the logger by fetching its configuration and
re-applying it later. In the current state, maybe_reset_logger/1 will only
re-apply part of the configuration (the config key), which does not include
other settings such as filters and formatters.

The effect can be demonstrated by adding filters or formatters to ~/.erlang
and running rebar3 shell on any project (without any --config argument, this
one reload the logger env; updating the kernel/logger environment in
~/.erlang fixes it).

For example:

logger:set_handler_config(default, #{formatter => {logger_formatter,
                                                   #{template => ["> ", msg, "\n"]}}}).

This patch makes sure the default handler is re-created with its entire
original configuration.