New features:
- You can monitor multiple webpages at the same time.
- Support multiple receivers.
- You can view logs in execution.log file.
Fixes:
- Fix the bug that error in monitor multiple webpages, threads can only be started once #2
Refactors:
- put the logic that get target page content into one function
- put the logic that init email util instance and get receiver email in a function
- separate the notify function from the website monitor