feat: export a utility class instance

[justgo97]

Hello, This PR wraps the current code in a class to make more room for customizations and more features:

• Get the current status of the update process ( updateElectron.isActive, updateElectron.isChecking, updateElectron.isError, updateElectron.isDownloading and updateElectron.isDownloaded )
• Adds a new function to allow the user to stop checking for updates ( updateElectron.cancelUpdateTimer() ) fixes https://github.com/electron/update-electron-app/issues/65
• Allow changing the dialoge messages (updateElectron.dialogTitle, updateElectron.dialogDetail, updateElectron.dialogButtonRestart and updateElectron.dialogButtonLater) fixes https://github.com/electron/update-electron-app/issues/116

This also keeps backward compatibility by exposing both an instance of the class and the old used method, Let me know If this is not needed then the old syntax can be deprecated and just the class instance can be exported. If these changes are approved then I will proceed to document the new additions in this PR or another.