no default for config.Pkg when using --config

[asphaltbuffet]

The example config documentation claims that the default for pkg: is slogx:

# the name for the generated package.
# default: slogx
pkg: example

However, that only gets set to default when not using a config file, as the default is part of the pkg flag. There should be a check in the constructor for a new config object that adds something there (instead of in the flag or in addition to the flag value).

There are no tests for reading a config file for pkg: ""

As a result, when creating the package, the call to os.Mkdir(cfg.Pkg, 0o755) fails with an unhelpful message passed to user:

❯ go run go-simpler.org/sloggen@latest -config ./.sloggen.yml
mkdir : no such file or directory
exit status 1

[tmzane]

Thanks for the report, fixed in #33.