Dedicated type `LoggerSection` turned out to be redundant since it doesn't
do a hidden logic and just uses `config.Config` API.
Remove `LoggerSection` type and implement `Level` which do the same.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Create `logger` sub-package of `config` package. Implement `LoggerSection`
type of logger sub-section. Add `Level` method to read logger level config
value. Default level is `info`.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
