Check for new version

[derekantrican]

Eventually, it would be nice to have a function that checks for a new version of the program from here (probably from the GitHub releases page?)

Something like this:

1. Every time the main function runs, it calls the CheckForUpdate function
2. CheckForUpdate will check the current version (a global variable in the code) against the most recent published version on GitHub releases page
3. If a newer version is found, we will check a ScriptProperty to make sure we haven't found that before. If we haven't, we will email the user that there is a new version (and set that ScriptProperty). If we have, do nothing