Config: Resolve log_level setting input value to Logger constant for severity level

[olleolleolle]

This is a feature request: for config option log_level, always convert given input to a Logger constant before setting the value.

The Config log_level setting currently does no resolving, but expects it to be a Logger-compatible level setting. It is used in .setup_logger.

Use-case: call the hutch CLI with an ENV variable set to HUTCH_LOG_LEVEL=DEBUG hutch

[michaelklishin]

This is a follow-up to #260.