Add support for log levels. Fixes #51

[klaudiosinani]

Description

The PR introduces the ability to display/hide logged messages through a scaled logging level system. The logging level can be defined through the logLevel option, which is of type String & it is part of the configuration object pass to the initialization of a Signale instance, and can be one of the following 5 strings:

• 'info' - Displays all the messages of all logger types.
• 'timer' - Displays only the messages of time, timeEnd, debug, warn, error & fatal logger types.
• 'debug' - Displays only the messages of debug, warn, error & fatal logger types.
• 'warn' - Displays only the messages of warn, error & fatal logger types.
• 'error' - Displays only the messages of error & fatal logger types.

The default value is 'info' for the logLevel Signale configuration option, as well as for the logLevel attribute of custom logger types. The option is inherited from parent to child instance whenever the unary signale.scope() function is used.

const {Signale} = require('signale');

const signale = new Signale({
  scope: 'parent',
  logLevel: 'debug' // Only `debug`, `warn` & `error` messages will be logged
});

signale.info('Info message');
//=>
signale.time('timer');
//=>
signale.timeEnd('timer');
//=>
signale.debug('Debug message');
//=> [parent] › ⬤  debug     Debug message
signale.warn('Warn message');
//=> [parent] › ⚠  warning   Warn message
signale.error('Error message');
//=> [parent] › ✖  error     Error message

// `child` logger inherits the `debug` log level of its parent instance
const child = signale.scope('child');

child.info('Info message');
//=>
child.time('test');
//=>
child.timeEnd('test');
//=>
child.debug('Debug message');
//=> [child] › ⬤  debug     Debug message
child.warn('Warn message');
//=> [child] › ⚠  warning   Warn message
child.error('Error message');
//=> [child] › ✖  error     Error message