MSG-526 Add support for per-module level overrides

This enhancement allows the configuration of module-specific level overrides such as:

      {lager_journald_backend, [
          {level, debug},
          {level_overrides, [
              {foo, notice},
              {bar, info},
              {baz, warning}
          ]}
      ]}

Given the example above, `debug` messages will be logged for all modules except foo, bar, and baz, which will only log notice, info, and warning or higher respectively.
s/ = /=/ for consistency with the rest of the file.