Add support for secrets/sensitive-info filtering

[klaudiosinani]

Description

The PR introduces the ability to filter out secrets/sensitive-info from the body & metadata, i.e. scope name etc, of to-be-logged messages. The feature can be utilized through the secrets option, which is part of the configuration object passed to a Signale instance on initialization. The option is of type Array<String|Number> and can hold multiple secrets, all of which are removed, if present, from the to-be-logged messages and replaced with the default '[secure]' string. Additionally, when the unary signale.scope(name) function is used, the returned Signale instance inherits all the secrets belonging to its parent. The secrets checking process is performed in a case-sensitive manner. Finally, this PR is a follow-up to #71.

It is critical and highly recommended to not type directly secrets in your code, thus the following example serves only as a simple & easily reproducible usage demonstration.

// foo.js
'use strict';
const {Signale} = require('signale');

// In reality secrets could be securely fetched/decrypted through a dedicated API 
const [USERNAME, TOKEN] = ['klaussinani', 'token'];

const logger = new Signale({
  secrets: [USERNAME, TOKEN]
});

logger.log('$ export USERNAME=%s', USERNAME);
//=> $ export USERNAME=[secure]
logger.log('$ export TOKEN=%s', TOKEN);
//=> $ export TOKEN=[secure]

// `logger1` inherits all secrets from its parent `logger`
const logger1 = logger.scope('parent');

logger1.log('$ export USERNAME=%s', USERNAME);
//=> [parent] › $ export USERNAME=[secure]
logger1.log('$ export TOKEN=%s', TOKEN);
//=> [parent] › $ export TOKEN=[secure]