Add isEnabled() API Method for Logger Enablement Check

[klaudiosinani]

Preview

Adds the ability to check whether the logging functionality of a specific Signale instance is enabled, by utilizing the isEnabled() method:

const signale = require('signale');

signale.success('foo');
//=> ✔  success  foo

signale.isEnabled();
// => true

signale.disable();

signale.success('foo');
//=>

signale.isEnabled();
// => false

Changes

• Exported isEnabled() as an API method. b3454e1
• Updated documentation. 5f7b6ff